The idea is that each bullet counts for a lot, but that doesn't mean you shouldn't pump them full of as many bullets as you can spare; they'll get a chance to shoot back before they die if you don't finish them off. Cover actually means something, it's better to make sure there's a pillar or a desk near by to duck behind, especially if you have to reload or bandage yourself. Combine that with the ability to strafe, ramp, ladder, and double jump and you've got yourself one hell of a good time.