People hated WotLK when it first came out because, basically, it was like the only raid version currently available was of the same difficulty as normal mode is now (in WoD) and was almost entirely a raid that was previously featured in vanilla. That is to say, it was jokeworthy. That said, everything outside of that major source of endgame content was excellent, and despite being easy and being a remake, Naxxramus was still an excellent raid. The complaints were, however, valid.
Ulduar came in 3.1, brought hard modes and outstanding content, and the world was made right. Then ICC came along and made this amazing expansion legendary.
People hated MoP for a variety of reasons, but the main one was gated daily reputations and valor rewards locked behind reputations, they dailied the shit out of you. Dailies were (and still are) fun when they are 1. Enjoyable and fun to play, and 2. Completely optional while still providing worthwhile rewards.
By the end of MoP, MoP had gotten the daily formula right, and designed new systems (Timeless Island, Flex raiding, cross-faction raid boss tagging, mildly challenging and mildly rewarding LFR, etc) that served the game extremely well. Sadly, Blizzard chose to throw some of that away, and unhealthily obsess on some of the others, as well as some confused rose-tinted look into WoW's past. And thus, WoD became a thing. A nightmarish, terrible thing. But hey, at least the dungeons are good.