1. The fact that Human-Players would pop up anywhere, when they join, instead of taking control of existing Marines.
2. The Crusher's dramatic yet boring entrance with unfitting music.
3. The migration to third person, whenever we deploy a sentry gun.
4. No animation to pick up guns. Also, after a cutscene, the player's hands (holding a gun) just spontaneously appear.
5. The painfully bright colors of blood. (See what I did there?
); Poor AI (Possibly worse than AvP2010?) because the marines stand there and shoot (while being clawed) and Aliens stand there and slap marines (while being shot at).