Suspense is one of the key factors in a successful horror movie. The viewer needs to be constantly wondering, "Oh no! What's going to happen next?" "Is the monster in that room? Or wait---is it that room?" Suspense can be created by having the main character stumble into a hall of doors with the monster behind one of the doors. As well, a clock ticking down can cause a movie viewer's heart rate to get faster. Suspense keeps a viewer on the edge of his seat, sweating over what is going to happen next.
Complementary to suspense is surprise. A bad guy jumping out from the shadows, in a spot previously unknown to a viewer, is what can make a viewer jump or scream. In "Psycho," for example, surprise happens when the main character is taking a relaxing shower and, suddenly, we see a knife come into frame.
A good horror movie builds in its horror. More factors come into play. The body count goes up. The villain invents a new terror-inducing trap. These things add to the terror a viewer feels. The stakes must continue to go up for a horror movie to be good. It makes the action scarier, and makes the viewer become more invested in what happens.
Some horror movies are scary, because there's that thought of, "What if that happened to me?" A forty-foot tall monster on Mars might not seem as scary as the spider that crawled into the bathroom sink in the New York City apartment. When viewers feel a connection to the horror, the events can feel more terrifying. It can keep a viewer up at night and checking around every corner.