Pretty straightforward, right? Inspired by this one conversation I had with Ray in the Hangout a couple of days ago.
So, what makes a game truly scary in your opinion? Personally, I find that oftentimes what cannot be seen has a greater effect on the player than something that can be seen. I'm also not the biggest fan of jump scares. Penumbra, a game that was brought up during said HO discussion, was used as an example: you're exploring this abandoned, underground complex in Greenland, surrounded by darkness with no way out, practically buried alive under an enormous amount of ice. You're given subtle hints: you know that there's something out there in the darkness. You don't exactly know what, but when it finds you, it'll probably murder the shit out of you. Or so you think. You'll experience a constant feeling of uneasiness, since the game does a great job at making the player's imagination run wild while there's nothing actually happening in the game itself. It also paces itself rather well, and for an hour or two, nothing really happens, yet you're given the impression that shit might hit the fan any minute, and you have nothing to defend yourself with aside from a rusty hammer. In the end, it just works really well.
Alright, I don't want to make this a lengthy post, so go on and start discussing stuff. What elements and design choices can make a horror game a truly frightening and memorable experience? What games did it just right, and which ones failed to get a reaction out of you? Keep in mind that I'm not asking you to just list scary games. This is not that kind of thread.