I can only explain why I hated Cata. None of it has to do with it being objectively good or bad, and is just how my idea of a good time was not catered to. I quit late WotLK, came back to check out Cata, and hated it completely.
They killed the Park. I was on an RP server for forever, and they essentially took the place where I did 90% of my playing and got rid of it without providing a replacement. I hear a lot of the RP moved to the Cathedral district but I never went back.
They stuck everything on rails. I level a lot. It's one of the primary things I enjoy doing. Things like forcing your character to go through all of the quests on their starter zone before they can even leave the zone, taking control of the character and taxiing it from place to place, making a bunch of quests that required mechanics outside of actually running around using class spells, and generally narrowing the focus of the world to the point where it felt remarkably small made questing the least fun it has ever been for me. I still don't know if it's possible to get to some of the cata zones without porting in to them from the world, though IIRC I ended up in that underwater zone after some exploration. I didn't like the fact that you couldn't go from zone to zone, and that each zone was so separated from itself that natural exploration was curtailed. This is also the first expansion where I never discovered where most of the instance portals were. Everything felt so disconnected.
Increased crap about heroes. This was already creeping in by WotLK but I am not a fan of NPCs acting like my toon is Elune's gift to humanity all the time. It felt patronizing, particularly when I'm at the early levels of the expansion. They haven't let up on that crap in later expansions either, but Cata was when it seriously started pissing me off, probably because it compounded the annoyance I felt every time I ended up on a quest where I had limited control over what was going on and felt I was in theme park ride mode.
Lack of community. This one is probably a major factor and should be higher up. I quit in WotLK. When I came back to Cata my server was essentially dead. I tried rolling elsewhere but could not find any place where I really enjoyed the people I was playing with. That made it really easy to stop when I hit cap, looked, around, and decided I hadn't had much fun up till then so I might as well stop.
They changed up the old content. This is pure nostalgia and I get why they did it but it doesn't mean I have to like it. I was pissed when I found out how much content was simply gone, how much had been simplified, and how little was left from the world that I had loved. It didn't feel like WoW anymore during that expansion.
Wrath had been fun. I enjoyed the mechanics, the raids, the way to gear up, the classes I played, and the zones. Cata was massive disappointment in comparision to that, so I walked away from it fairly quickly.