Ah horror games. A special class of games designed to make grown-up gamers pee on their pants, the ones that stay on your mind after dark, the ones you glad are not real, the ones that you don't want  to play in front of your GF.

So what makes a good horror games?  Below are what I think make a horror game good.

• story (including characters characterization), most and foremost, IMHO. The horror must be build upon strong and solid story. Eerier is better, preferably something unknown and unexplainable. Like the Silent Hill series and first Resident Evil, the next Resident Evil doesn't have the "what's this all about?" factor since all already explained in the first. They're still a great games but not "horror" anymore.
  A good horror games make gamers wondering something like : where's everybody, what's going on, hey is that a face I see on the wall, damn I hate walking in dark hospital corridor at night.
  I love how story being told in small notes, scratch on the wall left by the unfortunate NPCs.
  
• music, a good music does what good music does. To build the atmosphere.
• surprise factor, I mean real surprise. Something so sudden, it startle you. Whether it's a whisper, laughter or window-glass breaking,
• being alone, there's horror in just being alone and have no one to depend on. Although some occasional companion makes us crave for more and make loneliness harder to bear. Co-op in horror game? Bad Idea.
  
• no mass-murderer weapon, want to spoil horror game? Use rocket launcher, it's fun but it saps the soul of the game. If you still want to do so(if the option is available), do it after finished the game.

Have anything to add?

horror game