I think alien descriptions might be too harsh on player imagination, and therefore on the atmosphere (which, I agree, is GREAT). Char reaction on first encounters, on the other hand, could be incredible - especially if they're based on char stats/equipment. A HW Specialist with an AT Rocket Launcher should be glad to blast open a slow, well-armored target, while a hunter could set in him a slight panic. A scout would react very differently.