I quit with a few months left of Cata (summer 2012) mainly because i wasnt enjoying the game as much anymore and loads of people i played with had already quit.
I didnt like alot of the changes to the game during Cata and when i saw MoP i didnt like the direction even more. The game clearly being marketed at a younger audience with the cartoon kungfu panda concept and pokemon minigame incoming. After enjoying the dark atmosphere of Wrath and TBC the oriental cartoon pandas totally didnt grab me.
On a gaming level i didnt like the lack of challenge and the homogenisation of everything. The community had taken an obvious nosedive, it was very different and much more toxic during Cata than it was in Wrath.