Hey guys, I'm playing this on the PC and got to the part where you fight your first androids in the Marine Campaign. 
Frankly, I died in seconds, on Hard difficulty. I then went in again, this time 'prepared', ready to take cover. Seems there isn't a crouch button in this game.
Am I nitpicking here, or are you guys also bothered by the omission of something so basic for the FPS genre?
 

I think yeah that fight in the mono rail station is the first time i really noticed and missed the absence of an ability to take cover by ducking.  I disliked that part of the game too.
 
Cause against any enemies that are alien, cowering behind sometimes would only lead to a bad stuff. 

Let me third what Azure and RedHerb just said. Haven't reached the part with the androids yet, but the game is designed around fighting aliens and Predators. You stay still for more than five seconds (For MP, make that three) and you run the risk of either being shoulder cannon sniped from the front or insta-killed from the rear. Now think about it; how is crouching supposed to improve your game, exactly?
 
I can understand people missing iron sights (Which would have been useful), but everyone complaining about the lack of crouch hasn't been paying attention to the actual game. In this game, Crouch=Die Now button.