Remember, there are (at least) five different enemy races, and Bungie is still playing things pretty close to the vest as far as enemy types and combat intricacies. Also, in the E3 gameplay demo when Joe finally manages to get a headshot on a Fallen on the lagged out PS4, the message above reads, "Critical" NOT "Headshot" which basically confirms there is going to be some kind of differentiation between anatomy of enemies. Plus, this game is designed to be progressive, over years. This first "book" may be confined to 5 different, humanoid-ISH races, but if almost guarantee after a couple years all kinds of insane enemy types are going to be dreamt up. Can't wait for this!