1. Darkness/vagueness--who isn't still just a little bit afraid of the dark?
2. The unknown horror--what you don't see scares you more than what you do see
3. Missing, dead, or mutilated people or, to a lesser extent, animals
4. Sounds that are unidentifiable, smells described as unidentifiably horrible
5. Anticipation that something bad is about to happen--again
6. Stories/rumors that this has happened in the distant past
7. Unseen monsters (but not necessarily unheard, unsmelled, or unfelt)
8. The inability to escape your fate, or the perceived inability to do so
9. An intelligent monster, or one who at least appears intelligent from its actions
10. Limited time to for actors to react/act/plan between horrifying acts
11. Like in Greek tragedy, the violence occurs primarily offscreen, leaving the watcher to imagine the horrifying reality taking place
Everything I learned about true horror was from H.P. Lovecraft, the master even above Poe, who's darn good himself. Shelley's Frankenstein, Robert Louis Stevenson... All great horror/sci fi authors.
My favorite horror movie (which is also sci fi) is "Alien" (singular--the first in the series). It meets most/all of the criteria above.