If you don't know, and you very well might, Punching/stabbing/force pushing enemies in the face does bonus damage. I think it is a straight up 2x multiplier, but that may vary. Also, punching Vex in the heart might produce a similar result, though I find it harder to hit. Also, note that sometimes punching things in the face is kind of hit or miss, especially on faster targets which move or try to punch you when you are close. This may cause a punch to the face to 'miss' and become a body shot.