Congrat's your genuine curiosity brought me out of lurking!
Anywho, As other have put it, there used to be a pretty in depth aggro system in the game with lots of moving parts. It was a effectively this whole mini game where the group assisted managing the main tanks agro utilizing their tools so the main tank could spend as little time as possible in tank stance as it was a MASSIVE DPS increase to do so. Everyone popping their diversions and such, the ninja Shifting their aggro to the tank and reducing the healer/BLM's aggro, all of this at no cost to them as everything is a oGCD. All with the goal of letting the tank stay in DPS stance for damage. It was honestly really fun and engaging....
In Organized High end content
Outside of that, it was infuriating. The main issue with the whole system was that basically as a tank, your DPS was ENTIRELY reliant on your DPS's ability to use their buttons, and if you think people would just use them when asked BOY did people take offense to it. The thing is, alot of the enmity reduction skills were VERY powerful, you didn't even really need to time them in most cases, but in PUGs this was often too much to ask. If they weren't hitting their enmity reduction buttons all they really had to do was out DPS a tank to pull aggro, which they SHOULD be doing, even if they were sub par. Weirdly enough it this led to alot of very bad players getting very big egos, because they thought their DPS was too much for the tank to handle, when in reality they were just not hitting their "make the tank do more damage" buttons. You would end up having to constantly nerf your own damage as a tank (and in WAR case lose access to your defining abilities AKA Fell Cleave) Just because people couldn't be bothered to hit some extra buttons. On the flip side of this, as others have mentioned, you would get players who, if tanking, would be in tank stance ALWAYS, and it was NOT a small DPS loss being in tank stance. It varies from class to class but it was like 30-40% DPS loss, and the players who did this usually weren't very good in the first place, leading to some tanks doing ABYSMAL dps. You could hardly blame them for this conclusion since the game basically told you to do this, however in most situations it was entirely unnecessary and just caused things to get drawn out. If you see arguments these days about healer DPS vs just healing (please do not start this up here) it was basically the same thing, but with tanks. Long story short, the issue with enmity management is that it just divided the player base and got players mad at each other. Ideally from a tank perspective, you would do a single enmity combo (or less if a WAR in SB) and then play in DPS stance the rest of the fight. After the changes tanks now all play like they used to play in DPS stance all the time now, but also have the extra mit like they are in tank stance now as well.
Other issue some classes, like the mages also just had bad tools for handling it, mages only got access to lucid dreaming, which cut their enmity by 50%, this issue being that BLM and RDM tended to start off with a bursty opener, meaning if you didn't have a ninja, the tank needed to gain extra aggro. This caused a effect where most groups were basically locked in a specific DPS set up. you had DRG/BRD (or MCH if you were feeling like being different) because that combo was basically mandatory, the DPS was nutty, then you had a ninja because 60 second trick attack was absolutely silly and they came with aggro management tools that you effectively needed so your tank could deal more DPS. Classes like Monk tended to struggle in these days because it brought basically no synergy buffs to party, and that was the name of the game back then. Basically if a class couldn't nicely slot into a meta comp, it was considered bad back then, much worse then classes are these days, as it wasn't just raw DPS the group would be missing.
While I do miss the system I think it's for the best that it was removed, though I would like to peak into a game where they figured out how to make it work without it being frustrating. It was the cause of so much balancing issues, and and was the center ALOT of points of frustration.