I'm a fan of a lot of horror movies, but the ones the disturb me the most are the ones that involved psychos in masks.
For example, Halloween, there is no emotion, there is no face. If you somehow survived an encounter with him, you would always be looking over your back wondering could that be the killer, what about that person?
Compare that to another movie I saw recently called The Strangers. The trio of masked strangers were quite successful in terrorizing the couple. But then as the trio has the couple at their mercy, they remove their masks. Yes their faces will be burned into the couples memories for the rest of their lives, but it removes the blind terror they would have lived with every day if they kept the mask on.
After all, which impression is left on us more, that Hockey Mask which hides the face of a ruthless killer, or the fact that underneath that mask Jason is nothing more than Sloth from the Goonies.