What make a good scary movie?
My first rule is that the characters in the movie be afraid of the bad guy. Notice how in most horror films, the victims suddenly get all brave and actually go after the bad guy (killer, tormentor, demon, whatever). If they're not scared, how can you, the viewer, be scared?
My second rule for a good scary movie is the writting/acting. Again, notice how in most horror flicks, the stupid remarks and one-timers and the overall bad script (which always tries to be funny) is what makes them really cheezy.
My third rule for a good scary movie is the special effects and the way to show them on the screen. Most horror movies tend to have a low budget for special effects and, even worse, show way too much than they should. Notice in the first Alien movie, that we saw the creature for very short sequences and in almost total darkness. That, to me, is scaryer that if they would have shown us the more of the creature.
So here's the compilation:
1) Characters in the movie must be afraid.
2) Keep it serious and stop the cheezy one-timers.
3) Don't show too much, leave more to the imagination.
This, of course, is only my opinion.
Ultradan