Well I'm mainly a PvP player. I did PvE occasionally for fun with a group of friends. I'm really dissapointed in PvP changes. Stampede getting past alpha and taking 1 month to fix, ridiculous warrior damage, frost bomb, instant CC, new dispel system etc. Overall PvP balance is plain terrible. There are fixes coming next patch that should've been hotfixed right away.
The new upgrade system broke my came'ls back. That was the last straw. I really hated the forced daily grind for rep/valor. I got my VPs capped once, maybe twice during the 2 months I played MoP. You can argue here that dailies are not mandatory, but for any non 'casual' player that's unfortunately not the case. Also with this new change it takes 3 months to get geared with 3.5k conquest cap. Aka you can give up on your alts, there's absolutely no point to change char mid season anymore. The gap in gear is way too big.
Also one reason why I quit was because many of my friends quit also for the same reasons I ended up quitting soon after them. MoP had great potential PvP wise, but somehow they managed to ruin pretty much everything.
I've been playing almost since vanilla release. I had my moments, I have many fun, great memories. But I think it's time to 'move on'. It's such a shame, I will miss WoW.