wow, really guys? whats with everyone saying the enemies?
what about horror games with no enemies? only one enemy?
atmosphere (area and noises, on the poll)
and story,some people are even discounting the story, but without a decent story, a horror game is nothing more than a virtual haunted house, story gives you a reason to be there, a reason to keep going, and a reason to be scared of what lies ahead.
penumbra, even with enemies, you rarely get the chance to overpower them, and are forced to flee, in fear of their power, you also have a creepy story that evolves before you (not a single person wasnt worried about pushing that "ignite" button at the end of overture)
the sound is also different, because it doesnt try too hard to make things go bump, its generally quiet, which makes it really tense when you hear something out of the norm, penumbra could work just as well with no enemies, as the ones in overture are really nothing more than tools to force stealth
plus, the game atmosphere so well that when the lights turn ON, you get scared