Gore. Jump scares. Lovecraftian monsters. Creepy music. There are many tropes that make for a terrifying horror game.
The feeling of vulnerability and lack of control over what happens next is often part of what creates the most intense and frightening moments in games. Outlast and Until Dawn are two examples where neither offer much in terms of gameplay mechanics but strip power and control away from the player, leaving them with only one true weapon against all things evil: the quick and decisive decisions that they are forced to make.
Likewise, the sense of isolation, combined with the mysterious unknown and clever level design also contributes to generating a sense of dread. This is apparent in games such as Dead Space and Amnesia, where the feeling of being alone in the dark, in an unfamiliar and hostile environment where anything can happen at any given moment adds to this unsettling feeling.
The playable teaser for the recently cancelled Silent Hill is a terrific example of these elements. Walking down the same hallway over and over again slowly drives both the game and the player further down insanity lane. Each stage never truly repeats itself, which is part of what makes it so great and horrifying; it is unpredictable.
Unlike having something jumping towards the screen with the volume cranked up to eleven for the 748th time, never knowing what lurks around the corner and being powerless in the face of the unfamiliar can give an odd yet satisfyingly unnerving feeling. Fear of the unknown, as a famous horror writer once said, is the most powerful kind of fear.