I have several...
One is mercenaries. While they will forever be the straw that broke a lot of my friends' backs (leading to them quitting), I can appreciate how necessary they were (
are) to EQ's continued existence. The MMO playerbase that I was apart of 20+ years ago (Legends of Kesmai, Gemstone, The Realm, Meridian59, UO) has aged and fallen into the familiar trap of being slaves to their RL schedules. As a result of WoW's timely release, much of the "next generation" has grown up on games that require little to no time or effort to progress in. EQ needed to implement some way to allow progression without socialization in order to stay somewhat relevant. I may not like them but I've been boxing since PoP ("Log in for flags or give your info to someone who can log you in!") and some of my closest friends ingame were clerics so I never had to worry about needing a healer (in many cases, I was 'the guild' in the "Gotta go. Guild needs me" excuse
).
Beastlords. Hated this class with a burning passion from the day it was introduced. One of the reasons is because (in RL) I hate cats and 90% of the first batch of beastlords were cats. As a raider at the time, I also despised the class because SOE screwed loot tables so vendor purchasable beastlord spells were dropping off of raid mobs. So when we needed RGC or KEI or Elnericks or Garrison's, we'd get some crap like Spiritual Strength or something like that. We had a saying at the time, "Beastlords and their spells are a plague on this land". I didn't like pet classes in general because, besides the Necro, SOE always balanced them with their pets as their primary source of power. Problems with pet push meant that we banned BL pets from raids or just outright banned beastlords altogether ("MGB Paragon and then switch to a useful class"). About a year ago, a friend quit to play ArchLord (I think that's what it was) and gave me his account. Now, the primary characters on the account are a 90 berserker and wizard, both raid geared. But there was also a beastlord who, while modestly geared, was the correct race (Barbarian) so I took her out for a spin. Now, my favorite classes don't have anything to worry about and I'd never roll another one, but the beastlord has been pretty fun. It strikes me as a sort of melee mage with utility not tied to specific summons.
Instanced raids. I still hate them with a passion that burns like a billion blue giant stars. I believe that spawn racing made raiding much more visceral than the current convenient scheduling environment. But again, just like mercs, I realize that the playerbase is older and less focused and people would leave in droves the moment they lost out on a raid target to another guild that was faster, more organized, and more efficient. I miss the days of tracking a competing guilds' clerics to see how capable they were that day (you aint raidin' nothing of consequence with three clerics online). Most guilds nowadays have no idea what other guilds have as far as personnel, much less knowing the names of the entire class roster (knowing the names meant that even if they went /anon so they wouldn't show up on a /w all "guild name" cleric search, you could still find them).