The questing and QoL changes in Cata initially were a huge hit. It was a small subset of players complaining mostly about stuff losing its charm. But if you compare the leveling experience pre-Cata to post-Cata, the overall changes were amazing. It used to be a nightmare to level through Vanilla content, because the exp scaling was all wacky, and you had no clear idea where you should be going--the Hero's Call quests in SW and Org were the true MVPs of that expansion.
I think the problem with Cata was mostly for people leveling 80-90 or 85 whatever it was. The zones you went to were incredibly spread out and required you to take all sorts of scenic routes that were incredibly annoying unless you had the portal locations and such memorized. Also Cata--much like, but not as bad as--Lich King squandered a lot of potential, and I think Deathwing was just a pretty uninteresting villain. I think the way it screwed with the timeline was also an issue--you went from Cata Era in the Old World from 1-58, then Pre-WotLK Outlands, then Northerend Pre-Cata, then back to Cata in the new zones. It's all sorts of screwy.
Honestly at this point I wouldn't mind some of the timeline consolidated, stretch out the Cata zones so you don't even need to go to Outlands and Northerend, and add the dungeons/raids from there to the Timewalking events or allow people to solo queue into them for xmog runs. I kinda wish there was never a reason to do WoD content again either, but that's too ambitious...