For me it's two major things holding it back.
One, I'm one of the seemingly few that cares about the story and the lore when I play, and the MoP story just doesn't catch my interest like some of the previous expansions did. It doesn't even compare to the dark vibe of BC or WotLK. VotFW was so bad that it made me break my vow to do every quest in every new zone. I just couldn't take it. None of the others were anywhere near as bad, but still, the best zones in Pandaria were less interesting than the worst zones in Northrend, to me, and at best on par with the worst zones in Outland. Most of the old world, before and after Cataclysm, was more interesting to me too. The Horde vs Alliance conflict, which I absolutely love, played far too little of a role in the leveling process, so the only cool thing about it was the Sha. I haven't done the new dailies focusing on the faction conflict, but dailies aren't my thing anyway. I'm disappointed it's coming out in that form instead of the leveling process. So yeah, in a nutshell, the theme and whatnot of this expansion just isn't my taste, especially since the Horde vs Alliance aspect is through dailies and probably raiding.
And two, after leveling is over, all I care about is PvP. I don't raid, or do any form of group PvE content. I'm vaguely interested in them, for story purposes, but I don't want to deal with whiny people who treat it like a job, and I don't want to have to grind them over and over to be geared enough to do the next tier. So without any of that, all I have after leveling is PvP, and PvP is an absolute disaster right now. MMO is a pretty poor genre for PvP in general, so I've never had high hopes for it from a competitive standpoint, and every single expansion has gone downhill in terms of balance, IMO, but this was the final breaking point for me. Having world PvP back via CRZ is nice, but it still doesn't quite compare to the old server rivalries that could be built through places like Hillsbrad or the IoQ.
MoP may or may not be the best in terms of having the most to do, but most of it just isn't for me. I play video games for two reasons, competitive MP, and good story, and while MMO has never been a good genre for competitive MP, it was always at least fun enough to amuse me despite the flaws. But that's far too broken now, and with the story being the worst yet for my tastes, I can't help but think this was the worst expansion.