For me, the most annoying are the constant need to "retry" and reload the game to properly connect and atrociously long load times, hell sometimes it never loads. However, the game loads super quick without an connection.
Another, (not sure if handset dependent) sometimes when playing, the game just briefly stops tracking your left joystick, maybe too much movement? I'll be playing then just stop, I have to quickly lift then press again to keep moving. In a middle of a mob is the worse for this to then happen.
The auto-aim power up doesn't track properly with a spread weapon. Your relying on luck that it picks the right target.