I think what makes things scariest is me. For example, take system shock 2, a game I -cannot- finish. the first enemy you encounter is a zombie. He has a lead pipe, smack him twice and he goes down, and he's really easy to dodge, telegraphs his attacks significantly, too. They scare the -crap- out of me because firstly, I don't always know when I'll run into one, I'm convinced the game randomises spawns a little. Secondly, their voices. The way they beg you to kill them, or run away, it freaks me right out. The tension the game invokes in me is just too much, and so, I make the game scary. I just can't handle it. If you want to assign that a cause outside of me, it's because of magnificent atmosphere. The game exudes threat, and I respond to it.
Similarly, the scariest game I -have- finished is Dead Space. The monsters don't have the terror of system shock, because they're all inhuman, but the atmosphere of threat I feel, is preserved. The sound design is pretty masterful, because it always sounds like the monsters are scuttling around you.
So yeah, I guess the key component is atmosphere playing on a players mind.
Similarly, the scariest game I -have- finished is Dead Space. The monsters don't have the terror of system shock, because they're all inhuman, but the atmosphere of threat I feel, is preserved. The sound design is pretty masterful, because it always sounds like the monsters are scuttling around you.
So yeah, I guess the key component is atmosphere playing on a players mind.