I would say good character development is crucial. That seems to be lacking in horror movies lately. I personally do not care who gets butchered in most of the horror movies, and that is not how I should feel as an audience member.
I also think great special effects are important. The effects should either be very well done, or they should be subtle. If I don't believe what I am seeing, it ruins the whole movie.
The plot should also be new and well-written. I am tired of seeing the same old ideas or constant remakes. I want to see something original.
Most of all, I want to be scared. I want to have nightmares after I've seen a scary movie. If I walk out of the theater laughing, that was not a good horror movie.