For me it's a combination of the lore turning shit and the game becoming more and more easy.
The lore just went out of control and I hated how every popular WC character got killed. I hate that everything is more or less black or white. People are either evil or possessed or they are good. This is why the best part of WoW lore atm is the Undead and their struggle.
I didn't feel that my epic dagger was epic anymore with everyone and their mothers wearing it.
I just want more challenge and no, i don't want to do the same dungeons over again, but a bit harder. For me that don't count.