As Yahtzee said, and this is one of the few things I agree with him on, a good horror game lets your imagination do all the work. Fear is in the mind after all, this is why Amnesia: The Dark Descent is so terrifying, when you play it your imagination does all the work. You start scaring yourself thinking about what's going to be in the next room, or down the next corridor.
But too many "horror" games these days focus on shock horror rather than tension.
But too many "horror" games these days focus on shock horror rather than tension.