(some of this might be repeats, apologies in advance)
Also not OG but I vividly remember the HQpocalypse.. Sooooo many interesting crafts were able to be HQ'ed at higher tiers than possible today (or at least, it was easier), so many items (especially the NQ attempts) were much more abundant/cheaper... it'd be interesting to see an AH history that goes that far back. Lots of wealth was accumulated during this time, as well as during the transition period. Rumors still abound about whether moon phase/direction/etc. matters for HQ success rates, general break rates, skill-ups.
BLM was a bit more interesting back before the damage adjustment was made to make it more in-line with retail. Nukes could hit for a good chunk more than they do these days.
Fishing in Al Zahbi was all the rage for a while. You could readily catch fish that NPC'ed for the same as black sole, plus decent filler fish inbetween (so total gil/hr was increased as you caught something more often). There was an NPC right there, too, so you only had to walk a few steps to offload your inventory. I remember the stressful fun of riding the ferry out there, hoping that the lv99 mobs that occasionally spawned on the deck wouldn't come down and wreck the entire boat. Nas ended up nerfing the NPC rates and removed that vendor, though.
Speaking of Al Zahbi vendors, there's a good reason that your mog functions are generally un-usable there (or at least, it was a good reason at the time). Look up "Gilpocalypse" sometime. Back before my time, mog functions were available all over the place. Perhaps as a throwback to that era (and also incredibly conveniently, if you're doing a lot of sky farming), mog functions still work today inside Hall of the Gods on the way up to Tu'Lia. "I left my silent oil back in my mog" isn't a good excuse

Item duping (and abuse of exploits in general) used to be much more common and blatant, but a number of 'high profile' people have fallen under the banhammer, and abuse has fallen more into the background. Many of the most egregious exploits have been patched out, or at least mitigated - and for those that haven't been fixed, y'all should know that many of them are easier to track than you might think, even if Nas doesn't do anything about it for *months*. All it takes is giving him a reason to go look. I have some characters rotting in Mordion myself to prove it. LPT: Don't be dumb like me.
There were various silly exp 'camps' that used to exist that have been nerfed or obviated. Folks used to get great exp in capped areas and would exp there all the way to 75. It wasn't uncommon for people to be taking in merits while still only having lv100-ish skill on various weapons/magic/etc. The lv99 mobs in ToAU areas made for all kinds of interesting shenanigans, too. On a tangentially-related note, I'll take this opportunity to plead for pet mobs in non-ToAU areas to go back to the exp they used to have in retail... it'd sure make soloing BLM up a lot more fun, especially considering how little demand there is for them in exp parties before the manaburn parties in the 50's.
Worms used to be able to move and chase you, which was hilarious. Nas eventually took away their track shoes, so they can't run anymore.
Some ENMs used to work-ish. There was a time when people were NPC'ing maneaters while farming galateia/viridian urushi/etc. That's where the materials for various blessed/hachiman/etc. items came from, and why they're slowly getting more and more valuable (people that have some of the few remaining are quitting, getting jailed, etc., so the items are getting more rare). It's also why Clothcraft is such a huge bitch to get to 100 (though, it's debatable whether it's even worth it getting it past 99 anyway, I suppose).
It wasn't terribly uncommon even back when I started a couple years ago to find various NMs up and running around, even NMs that dropped items that sold for decent loot. Other NMs had very little (if any) competition. For many folks, it was much more efficient to just wait and buy the item off the AH rather than killing the time to farm it and get the drop themselves.
Drama used to be much easier to find, especially back when things were getting changed drastically with the claiming system/etc. At one point, people 'figured out' the auto-claim system, and there were a handful of characters that would essentially be guaranteed a claim if they happened to be within aggro range of the mob when it popped. There are interesting drama stories around how the auto-claim system even came into existence in the first place. Most of this was before my time, but I'm sure many veterans can remember.
Kotaku ran an article about Nasomi once. This was one of the things that turned me on to Nasomi in the first place:
https://kotaku.com/final-fantasy-xis-pu ... 1820807418