Well, I didn't realize it until after the aliens. It took me until Warhead to figure it out.
Aliens are better than trying to play Crysis stealthy, that's for sure.
Also, Warhead seems to be a lot more lenient about grabbing guys. Crysis won't let you do it unless he's looking straight at you on a full moon, while you make a goat sacrifice to the dark god Yzzzlpz'arl.