Overdone jumpscares are what put me off. In my opinion, good horror games mostly rely on jumpscares and an eerie atmosphere that provides the necessary tension. Great horror games don't rely as much on jumpscares (although they are present) as they rely on tension, symbolism and the inferiority of the protagonist(s). Plain bad horror games rely mostly on jumpscares and gore. Let's get something clear; just because a game has jumpscares, doesn't mean it's scary. The case where the monster of your horror flick appears out of that door you were about to open causes the same kind of reaction you'd make if your cute little bunny pet jumped out of his cage unexpectedly. It's not as scary as it's startling, and if you expect it to happen in your second run through the game, you'll make it out without even so much as flinching.
Another thing that ruins my horror experience are action segments, especially in games which are marketed as "survival horror" like the recent game, The Evil Within. In a game like this, you expect supplies to be as minimal as possible, but no. Early on the game, you get a handgun and then as it progresses, you equip yourself with some electric crossbow, incendiary crossbow, shotgun, sniper rifle. If you think you'll ever run out of ammo, don't worry. There's plenty you can find lying on the ground or those crates you break with your bare hands. You also have those escorting segments where your friend does something while you have to protect him from waves of zombies, which is so not horrifying. Did I mention people still defend this game?
Shinji Mikami; father of horror, my ass.