EDIT: All of the mistakes and tips have now been compiled into a wiki article. If you add any more mistakes and/or tips please edit the article as well before posting here.

I wanted to share some stupid mistakes that I made as a brand new player so others can learn from me instead of making the same mistakes again. I'll edit as I make more
Feel free to add your own mistakes from either when you were a brand new player / if you only played retail after all the recent changes etc.

Inventory:

• Not stacking items - I didn't know that items can be stacked so I ran out of inventory space really quickly, ended up dropping a lot of useful items just to make room for others. Most items can be stacked (usually up to 12 per stack) and each stack counts as a single item. Click sort-->auto to auto-stack everything.
• Not knowing about the mog house - Even after stacking I ended up with 30 items really quickly and started dropping stuff again - because I didn't know about the mog house. Items that are not needed at the moment can be stored there to clean up carried items.

Gil:

• Not comparing prices before buying - A lot of times the same item can be bought from both AH and NPCs. If it's available from NPCs in a certain town it's usually cheaper.
• Not comparing prices before selling - Same thing. In most cases items will sell at the AH for a price way higher than NPCs. It's always a good idea to first check how much NPCs will pay for the item and then check the price history at AH. I didn't find many cases in which selling to NPCs was better (I would note here that if the difference in price is very small I usually NPC because they pay right away).
• Not comparing stacks vs. individual items at AH - Items can be sold either at a full 12 stack or a single piece. These count as different items at the AH and have different prices. The price for a stack does not necessarily have to be 12x individual item price (and usually will not be) so checking the difference in prices is a good idea. Same goes when buying.
• Selling needed items - I once sold an item at AH only to buy it again a few days later because I needed it for a quest. There are enough wikis documenting virtually what any item is used for so it's worth checking whether the item may be needed in the near future.
• Buying unneeded items - If you just level from level 1 to 2 don't rush to spend gil on level 2 gear. Gil is not easy to acquire for beginners so money should be spent wisely.

Mobs: (aka monsters)
Note that I started in Bastok. For Sandy / Win names of mobs may be different but the concept is the same

• Not using /check - Blindly attacking a mob without any assessment of how difficult it is is a bad idea. Generally the only mobs that can be used to get that precious level 2 are the mobs right outside the town walls (for Bastok these are huge hornets and tunnel worms). They are the only mobs guaranteed to be level 1 so they will check as equal match. Anything else is unwise.
• Assuming that all mobs of the same type are the same level - I took on ding bats and defeated them easily so I assumed that like, the hornets and worms, they are also level 1. That specific mob probably was but the next one was not and I found out the hard way. /checking a certain type of mob once is not enough, each individual mob has to be /checked separately.
• Not knowing which mobs aggro (i.e. attack you without you attacking them first) - Even if a mob checks as very tough, if it's a type that never aggros you can just walk past by it and it will ignore you. If you identify it as a type that does aggro then sometimes you'll have to take long detours.
• Not knowing about links - Linking means that when some mobs see you fighting another mob of the same type they will join and you end up fighting two or more mobs at the same time, e.g. by level 5 you can probably take on a vulture but two vultures at the same time will probably be too much. It's a good idea to check which mobs link (wiki!) and before engaging any of these, check that there are no other mobs of the same type in the area.
• Underestimating different zones - When you're level 1 you're stuck at the few square yalms (that's the standard FFXI distance measurement unit) outside the town walls. When you're level 5 you can take a few more steps out (Gustaberg are in my case). By the time you reach level 12 or 13 you can defeat pretty much everything in that zone - most mobs will check as too weak and the rest are weak enough that you can roam around pretty safely everywhere. At level 13 in Gustaberg I could take on 4 Quadav at the same time and still have over half of the HP bar left. That can give false confidence as you assume that the nearby zone (i.e. areas) will also be relatively safe. Then you head to the next zone and a couple of minutes later you're dead. Note that even mobs that are a type you already know can be different levels in different zones.
• Underestimating easy prey - That term is really misleading. The only thing that /check does is compare the mob's level to yours. It does not take all stats into account and does not take gear into account either. Attacking every easy prey mob is pretty stupid, especially when your gear does not suit your level, especially if the mob seems to have high defense and especially if it links.

Missions:

• Taking missions too early - The first mission (at least in Bastok) is really easy, basically take an item from NPC A and give it to NPC B. It does not take you anywhere near a mob that aggros so it can be completed even at level 1. Again that can be very misleading - while the second mission does not directly involve a battle, it takes you to an area infested with goblins, who do aggro and link and battles cannot be avoided. Completing the mission solo is not viable before at least level 10.

This is a good list for super new folks.

One other thing, that feels relevant everytime I'm in the Dunes or Qufim. Level syncing your equipment, and especially your weapons, is a bad idea. Have appropriate weapons, and when possible a cheaper piece of armor is better than a more expensive option synced down.

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Thanks


Yeah, that was also a main mistakes, though it mostly correlates with item stacking and mog house. Beastmen seals can be stacked as high as 99 / stack, so they really don't take a lot of space. Furthermore since they only become useful in more advanced stages, they can be safely kept in the mog house, however if you don't know about the mog house or about stacking and you think you're limited to 30 items total that's how you end up throwing them away.

Two more things came to mind:

• Not maintaining sufficient buffer - A "buffer" is the amount of exp earned since the last level. Being killed means losing exp and if the buffer is not sufficient to accommodate for the loss then it can also lead to losing a level. It's important to strive to maintain a buffer large enough to allow one or two deaths without losing levels, especially if the last level received unlocked new abilities and/or ability to equip certain gear or spells. This may be more easily said than done but at least in low level it's not very difficult. The only important thing is to never quit grinding as soon as the needed level is acquired, always grind a little more to build buffer.

And of-course I didn't even think about partying mistakes, and there are quite a few, so -

Partying:
Partying (if you just came from outer space) is a group of 2-6 players working together to defeat a mob that cannot be defeated by a single player of their level alone. The exp gained from defeating the mob is shared between the party members. After a certain level this is the only real way of gaining more exp.
Here are the mistakes that either I made or saw others make so far -

• Not setting home points on the way - A home point is the location where you return when you're killed. Most people start partying in a zone called "Valkrum Dunes", since it has relatively easy mobs (for parties, not solo!), however you first need to get there and since you're traveling through zones that are way more difficult than what a new player is used to (see my first post) it's very likely to be killed at least once or twice on the way. Having to start all the way from the beginning is not fun, so it's recommended to change the home point to the nearest one possible as you go through. The home point can be set at towns, outposts and sometimes checkpoints near the edge of zones.
• Level not being high enough - The lowest level I've ever been in a party with was 12 and usually that's also only if there's no one else looking. Unless you have a friend or someone who intends to power-level you, going to a party zone without being at least that level means not being invited to any parties and having to go all the way back. It may be a little boring but a new player has to grind solo for about a day before going to party zones. I personally grind even a little more than others, up until level 15.
• Joining bad parties - At least in the dunes you don't have to be really strict about the composition of parties, yet there are still two roles that are immutable - tank and healer. The tank is the member that focuses the attention of the mob and takes nearly all the damage. It has to be a melee job with a high HP pool. The healer is usually a white mage (though in low levels can also be a red mage) that heals the tank (who will need a lot of healing to survive the constant damage) and occasionally other party members if they happen to be hit severely. The healer needs to have a high MP pool because those MP are what's keeping the party members alive. As long as the party contains a good tank and a good healer it should be fine (at least in the dunes), but a party with no dedicated tank and/or no dedicated healer is doomed to get wiped (i.e. have all members killed). Better not to join such parties and even leave if already joined and no changes are made.
• Taking on the wrong mob - Just like in solo, mobs have different levels and the party should take the ones that are just right for its own level. If the mob's level is too low then hardly any exp is gained, if the mob's level is too high the party is wiped. Search wikis and the forum to know which mobs and areas are relevant for which levels.
• Camping close to aggressive mobs - A camp is where most of the party members stand and where the battles take place. The worst place to camp is close to a though mob that tends to aggro. I've had more than one case where we partied vs lizards but came too close to a goblin who wiped us all. Always check the surroundings on the designated camp.
• Pulling too quickly - Pulling means one person performing the initial attack of a mob then luring it to the camp, where the rest of the party is ready to battle. Usually you'd want to pull as quickly as possible to maintain a high exp/hour rate, however if the puller is too eager and fails to notice that party members are still seriously injured from the last battle, that the healer has hardly any MP left or just that someone asked for a break to go to the toilet - that means certain death. I cannot stress this enough - Pulling recklessly is the #1 reason I got wiped so far.
• Not watching for links - Just like with solo, in many cases the party is able to deal with one mob but not with two or more at the same time. If the mob type is known to link it would be wise to check that there is no other mob of the same type closer to camp (than the puller's target) so nothing can link with the target as it's pulled to camp.
• Lack of communication between party members - The party chat is there for a reason. Sometimes even with good intentions people fail to notice the healer's low MP or the fact that someone is poisoned or anything else that can affect the party. Make sure to notify members of potential problems and watch out for what others say.
• Not setting proper macros - If you don't know what macros are then read about it. Basically they allow you to set custom key combinations on the keyboard to perform custom actions, e.g. hitting the mob with a weapon ability, curing the tank etc. Party battles almost always require split second actions. If you go through all the menus you will definitely be too late. Make sure not to begin pulling before all macros are set and ask for breaks to change macros as needed (if you don't get that then that's reckless pulling).
• Not having everyone set a nearby home point - Part of partying means that occasionally (hopefully not very often but still) people will die. It may be possible to find mages that can raise dead players (usually high level white or red mages who came to PL friends) but sometimes there aren't any and it's necessary to start again from the home point. If it's really far away it means that the party has to wait a really long time for that member to come back. Depending on how long it takes they may or may not bother. In the dunes there are two possible home points - At the outpost and in the town of Selbina.

You can also trade your seals to the NPC in Jeuno, and not have to hold onto them at all.

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Level one does not simply walk into Jeuno

I mean... you definitely can, but I just meant in general.. Folks might not realize there's an NPC that will hold all your seals.

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Well done Nightcoder!

I am sure this will help a lot of new players.

These are pretty nitpicky and might clash with people opinions but anyway;

Use your correct equipment:
If you've started off as a Warrior you'll have a free sword in your inventory, ignore this and please please please obtain a Great Axe ASAP instead as it's their best weapon type for a good while and it'll make you shine as a player (Even if you can't afford one please just ask!).
I've come across so many Sword/Shield warriors which simply don't contribute much to parties as they think they do because they deal terrible damage. Yes you'll be expected to tank, you can still do a great job with the axe!

If you've started off as a Thief or decided to play one, please leave the dagger at home and take up either a Sword or Hand-to-Hand weapons until about level 33. Daggers deal absolutely dreadful damage all around and waste of a party slot, whereas Swords (Sneak attack Fast Blade) and H2H (Sneak attack Combo) deal amazing damage and will make you the party MVP!

If you're playing any kind of front line fighter job, please bring any kind of ranged weapon be it a Boomerang or a Bow/Crossbow with ammo no matter how crappy it is. Anyone and everyone should be expected to pull monsters back to camp, and it makes things harder if no ones packing.

Know your role:
If you're playing Red Mage you'll probably think it's really cool that you can fight and cast spells at the same time, hold that thought and put the sword away when you join a party and pull out a wand because you're going to be healing your party and debuffing the monsters instead.

If you're sword fighting in a party you'll be dealing terrible damage and not contributing much, plus while fighting it means you aren't resting to recover your MP which means slowing the party down. If you want to fight and cast spells unlock Blue Mage later on and have more fun!

Red Mage: it's fine to level only one sub until your RDM is 75. But this sub has to be WHM, not BLM.

While it may be true, you cannot do that right away since you only start with 10 gil. You will have to do the first battles with whatever you started with. It shouldn't be too much of a problem though, if the hornets were too difficult for default gear no one would be able to reach level 2 at all. The default gear was fine for me until level 4-5, after that yeah, must be replaced.
I actually started as RDM and what you've said is absolutely true. I'll even generalize that a little bit -

• Assuming that battles solo vs. party are similar - This is just not the case, especially if you're a mage. When you battle the very first hornet, even if you began as WHM (that's the job least suitable for solo play) you will have to do some melee. When you join a party for the first time your role will change significantly. Mages will never draw their clubs / staves / swords (RDM) when in a party, it's not their job. Each member of the party has a specific role (e.g. tank, DD, nuke, support, heal) and must stick to it, otherwise the party is likely to get wiped.

And also some other things that just came to mind -

• Level sync too low - Level sync means that a party is restricted to the level of a certain member; any party member whose level exceeds that is temporarily reduced to that level. All stats are downgraded to that respective level however the exp gained from defeating mobs is also according to that level, which means more exp from lower level mobs, so most mobs will sync to the lowest member, however sometimes this may be a fatal mistakes. A level 20 party can easily take on a fly or even a goblin but if it's synced to level 15 that would not be the case. It's better to get less exp and not get killed.
• Selling lower level gear - When the party is level-synced any gear that requires a higher level has its stats downgraded - and usually will perform worse than native lower level gear. Keep that level 12 gear for a while. Only think about selling gear that is suitable for at least 10 levels below, since that is the maximum gap allowed for level sync. Even then it may be a good idea to keep it (if you're not short on gil or storage) for when you change jobs.
• Hitting wrong keyboard buttons - Watch out those macro keys. Casting "Cure II" on the wrong person, who already has nearly full HP, instead of the tank, who just got a critical hit and by doing that wasting a lot of precious MP and cast time can mean death. If your keyboard is not comfortable enough, get a better one.
• Heading to difficult zone too early - Sometimes you may join a high-level party and members will want to migrate to a more challenging zone (e.g. leave the dunes and go to Quiffin). Don't go before you're ready. The worst thing that can happen at that point is if the party disbands for any reason (sometimes people have their own lives outside FFXI) and you're stuck in a zone way more difficult than what's suitable for your level and no safe way of getting back. If you really want to go then my two cents are that you intentionally don't change your home point. That way if the party disbands, worst can you get killed and lose some exp but then you can warp back to safety, however if you changed your home point to the same dangerous area and really have no way of traveling back you'll be stuck where you are and will have to shout until someone helps you.

Jobs:
[*] Assuming your job is for life - Your job is not your race and you will not be locked to your initial job forever, in fact you can change your job at any point simply by talking to your moogle (you'll find him in your mog house). There is no limit on how many times you can change jobs or any consequences, except that each job has to be leveled separately, e.g. when you switch to a brand new job (that you've never played before) you'll have to restart from level 1, however when you switch back your old level is retained. Sometimes it may actually be a good idea not to start as the job you eventually want to be. A good example - a WHM is one of the most needed jobs for a party (every party must have a good healer) but it's a terrible job to play solo, so the initial leveling is going to be difficult. Starting as another job (one that solos better) then switching to WHM and setting your old job as subjob (see later about that) makes things way easier.
[*] Not getting a subjob as quickly as possible - A subjob (aka support job) is a secondary job that you can benefit from while having another main job, though it has some restrictions (mainly the subjob level cannot exceed half of your main job's level). Note that there is no cons whatsoever for having a subjob so you will want to get that as soon as possible (that's when your main job reaches level 18). Of course you will have to level your subjob from scratch so it's back to your hometown and the hornets for a while, which may be really boring (though it will be easier than before) but this is required because you won't be invited to many high-level parties if you don't have a subjob. Level 21-22 parties generally expect most members to be already past that hurdle.