I'm still of the opinion that people are leaving the game because, no matter how much content they throw at us, most classes feel like garbage to play and are so homogenous that you can use the same keybinding scheme for most classes.
Everything is too slow and too boring. It's not even the number of abilities - it's the lack of unique utility and the unpleasant delay in reaction time.
Like, The Witcher has two basic melee attack buttons, but the combat is far more engaging because you have to make decisions and react quickly. You have to match your abilities to the opponent - Do you stun them? Are they weak to fire? Maybe they need to be contained in a magical trap to even hit them?
The only time you have anything like that in WoW is in PvP, and the GCD makes it that much more frustrating to engage in. It's like two people playing rock'em sock'em robots with a time delay.