Anyone can do shock value. Develop enough tension and cue the music right, then have something jump out: It's almost impossible not to jump in your seat. But that doesn't leave any effect on you when you leave the movie theater. To me, the best horror is psychological horror. The Exorcist, The Shining, The Omen, things that kind of stick with you long after you've seen them. It's what you don't see. It's letting the audience think a little bit, not spelling it out for them. Giving them credit for using their own imaginations rather than sticking in gags and tricks.