I played WoD for grand total of about 10 days, so I can't really talk about the raid content, but for me there were multiple ways the expansion failed for me:
1) Itemization - it is quite literally just +gooderer and the only number that mattered was ilvl. There was nothing interesting, no item I had to think about how to use, just every once in a while I got a purple one instead of a blue one because I was a good boy. That's a huge failure in a game that is all about acquiring loot and gearing up. Similarly all customization with jewelcrafting/enchanting was simplified or removed.
2) Phasing out of old content - After exhausting my 1 lvl boost I tried leveling out some of my old characters who were around lvl 80. Cataclysm mobs died so fast it wasn't even fun to kill them, since they were just bumps on the way between quest NPCs and MoP wasn't much better, I can't imagine what the difficulty is like for people decked out in heirlooms, I guess they are busy soloing dungeons or something, but it says something about Blizzard's confidence in their old content when they try to make you ignore it.
3) Dailies - some are fun, some are not. There's a reason why people remember TBC dailies so fondly - they were challenging and rewarding, since they came at a time where money was an issue for a lot of the players, since epic flying mounts were crazy expensive and heroics were bloody hard, thus giving players an alternate options to gear up, obtain materials and money for the content that was open to them but still challenging. The problem is that companions trivialized a lot of the WoD zones, which means you were running around trying to slaughter as much shit as fast as possible, the dungeons trivial and raids, at least at launch, were fucking terrible, thus leaving them as a timegated content for titles/hats. To add insult to injury, most of those dailies didn't feel that much different from the quests you sent your NPCs on, leaving you to wonder why the fuck are you even bothering and why can't they do this too.