I never found the game itself scary, its attempts at horror are too predictable to induce fear. in fact when I first played it with a couple of friends I "ruined the scary" by calling all the horror cliches before they happened. (eg: long hallway, lights turn off and I call the oh so predictable unseen monster roar etc.)
but, in contrast to horror games like amnesia, the gameplay is actually thrilling. I actually feel like I'm fighting for my life rather than simply cowering and waiting for the monster to go away. Fighting tooth and nail to NOT DIE is a much more exhilarating experience than just psyching myself out when the monster only shows up for 30 seconds once every half hour.
The fighting and imminent horrific death should a wrong move be made are what made the game enjoyable and gave it a pleasing tension.