Most hate is just a forums thing. People who enjoy stay and play. Those who come out here to whine about it are often just too picky and entitled. Every expac has a less than stellar moment for someone, but since we ALL have a different perspective, what sucked for them might be great for me, and vice versa. Honestly, as long as flying is unlocked, every expac is a great expac cause now I can skip to the stuff I want to do, and not all the other stuff I have to do just to unlock flight

My list would look something like:
Classic: Farming shards is horshit!
TBC: Why do I need rep just to run heroics?
Wrath: Man, fuck that troll zone and just bring on Arthas already
Cata: Deathwing is about to roll..... DAMNIT
Pandaria: Thanks for fucking up the Vale Garrosh, and too many damn dailies and reps.
WoD: Everything.... just everything
Legion: Borrowed power... again. So tired of farming powers
BfA: Well, this necklace sure has a lot of borrowed power. Feels familiar
Shadowlands: So no flying in Oribos or Korthia, and the only way to Oribos is a portal or a long ass flight
Now, from that list, someone somewhere will have nothing but positive comments for everything I disliked. It's just how things work.