Remember, the problems didn't start with Cata, MoP or WoD. They started with TBC.
WOW Vanilla was already the beginning of Blizzard rushing out a game rather than fully polishing it. As we know today, many horde zones were rushed out because the time was up. The same thing happened throughout TBC, that is why the TBC zones are completely devoid of landmarks and PoI. Blade's Edge mountains? Ugh. What a boring, 1 color zone. Instead of being rushed out like some vanilla zones however, those zones apparently were planned to be finished in considerably less time to begin with.
Flying Mounts obviously are a controversial subject, but the effects it had on the world can hardly be denied. The moment you get yours, travel is irrelevant, you don't even bother with flight masters anymore and the idea of "dangerous zones" goes away completely.
Also in TBC the dungeon design of Maraudon-style dungeons was completely abandoned and the short tunnel dungeon meta took place. Did you run the first hellfire citadel dungeon? Congratulations, you've run every single TBC dungeon. They all go the same: Press W, kill boss, Press W, kill boss, Press W, kill final boss. Dungeon over, GG. But why is that? Well, my guess is you can shit out 10 tunnel dungeons in the time it takes you to design one Maraudon. Which is why TBC has so many of them, but are is any of them memorable in any way?
Also TBC immediately rendered a ton of old content completely obsolete. Strat Scholo and the entire Blackrock mountain? Irrelevant. Raid zones? The loot doesn't hold up to level 65+ blues. Not worth the time anymore. Of course that was "just" all the old level 60 vanilla content, but every expansion did the same thing. WoW today is 80% dead content.
TBC was still wildly succesful, because those issues were nothing compared to what SOE pushed on people in EQ and we hadn't forgotten just yet. When you come out of an abusive relationship after five years, it doesn't really bother you that much when the new SO shows a few minor bad habits after a year or two.