The 3E sorcerer, the Greyhawk thief ... possibly the UA cavalier, although that wound up removed from the game. I don't recall the 3.5 warlock catching quite so much flack.
(And the 4E warlord is obviously the one you think people are thinking of.
)
Actually, the primary one I was thinking of was
dragonborn. But yes, the warlord was up there as well.
The Sorcerer also fits into this space, as some of its seeds go back to 2e, it was controversial among some groups across 3e's run, and only really gained full acceptance in mid-to-late 4e, about 20 years after the initial impetus. Warlock managed to ease, but not eliminate, this stuff because it was initially from a secondary late book and then flew under the radar when the Warlord wars sucked all the air out of that particular discussion.
You can even go back further and find stuff
like this about things like specialty priests (which evolved into the Domains we now know and which are seen as Absolutely Mandatory in many cases), or the constant back and forth about whether the Monk "belongs" in D&D or not. (Consensus seems to have fallen on begrudging acceptance--"I don't like it, I wish it weren't there, but I guess it isn't
harming me by being there.")
But that general pattern of things taking like 15-20 years to go from "torches and pitchforks" to "eh, fine,
whatever, I guess it's in the club" is a big part of what fuels the fire here. D&D rose from roots that explicitly broke molds constantly, that invented whole new ideas out of nothing more than "we need a class that suppresses undead because
Jim and his
vampire are being large keisters" or that threw rayguns and power armor into an adventure because it Sounded Cool. But within even a decade of it starting, you already had traditionalists actively trying to gatekeep what was and wasn't allowed in to the D&D Thematics Club.
The irony of course is not lost on me, bitter though it may be. The hobby that champions how open it is, how it contains the one thing a CRPG can't, namely the endless well of human creativity, is full of folks not only ready, but fervently determined to eliminate anything which does not conform to the Right Way Of Understanding this game we play. It's not even about badwrongfun; it's about badwrong
style. You can't play D&D with badwrongstyle--you might
corrupt it somehow, might in some unexplained (and probably inexplicable) way
take away what others enjoy just because you got a new thing you enjoy.