Pre MoP I loved dailies and running dungeons, raiding and pvp, pretty much anything in the game I thought was fun even just farming when I had nothing to do. Now, not so much, I can't say why or what exactly changed it for me but Mop is horrible for me, worse then Cata and it seems to only be degrading as time goes on. I find myself not logging in for almost a week straight when before I could not go a day without logging in WoW.
Overcompensation for an extremely poor previous expansion.
Frankly, after the last expansion I feel that they grabbed at every straw they could and tossed them into the game. They added in a ton of stuff and made it a veritable grind-fest to be on par with the rest of the group which does nothing but cause burn out. Yes, the content, "Doesn't have to be done," but to not do so gimps yourself and your raid. Blizzard is naive to think that the "optional content that offers quality rewards" is optional in terms of social stigma. In terms of actually optional, sure, if you don't mind gimping yourself and your raid by not getting the easiest and best available gear.
But why fix something that wasn't broken, e.i. vanilla and bc...
Well from my point of view that assumption is just wrong, I feel that, looking back, so much was broken or needed fixing and that the game is actually better than ever before. Of course the freshness is gone, but the only way to change that is make a whole new game, and tbh every xpac did quite a good job at refreshing the game for me... even Cata.