Bunch of different things I imagine.
They fixed stuff like "dungeons being too difficult" back in Cataclysm. Except nowadays, dungeons are basically useless for gearing up and are supposedly replaced by scenarios. Now, heroics scenarios actually offer some decent rewards from time to time, but it's still a complete rng even worse than dungeons. And not a single 5 man was added because it's supposedly such a huge effort that it would require them to cut out half of ToT bosses. Even assuming it's true, plenty of people still don't buy it.
Then we get dailies. I don't know which brilliant mind decided to lock everything back in 5.0 behind dailies. Crafting? Do dailes. Want to spend VP? Do dailies. Want to make +300 stat food? Do dailies. Want bonus rolls on loot? Yep.
I don't comment on raids since they're kinda-sorta-fine, plus I doubt they are nearly as important as some people believe. Yeah, I'm happy with my heroic modes, some people aren't - it's still a drop in the bucket anyway. Flexible raids might be a big thing in 5.4, we'll see. It's probably far too late, they should have been introduced back in Cataclysm after pugging got killed by increased difficulty and shared lockouts.
Oh, and let's not forget that you can only do 1 bonus heroic 5 man every day. It was quite nice to do bunch of them when you had time back in Cata, but in MoP? Nope, that would actually be convenient! Can't have that.
In fact, "convenience" - or rather, lack of it - seems to be a big thing in MoP. No flying for alts. No cauldrons in raids. No mass summon. No chaining bonus heroics. No 5 mans for quick gearing. Lesser charms solely through dailies - until recent patches. No reputation from dungeons - until recent patches and even then it's a small amount.
It's like a theme in MoP. 5.0 broke a lot of things which have slowly been fixed. But it was too little, too late.
Plus, I'm totally ignoring stuff like F2P games being stronger and offering a good alternative to P2P. This could very well be a reason that makes other one suddenly become far more important than they were before.
Edit: Oh yeah, and ghost servers. Having bunch of super servers and dozens of dead ones is hardly a good thing - and not everyone want to pay even more $$ to transfer. Virtual Realms might help with that one, we'll see.