Amnesia has such a tense and strong atmosphere that I'm able to be completely immersed in the game. The sound design and visual effects that indicate your fall from sanity keep you on your toes. You're often acting on instinct rather than knowledge of the game's mechanics or design. You're not trying to win - the game states this explicitly from the beginning - but rather survive in this hellish world.
Games that put the focus on their world and integrate game mechanics deeply into the environment and story are able to immerse me and are more memorable than games that hook me with scores and special abilities that are extrinsic to the game world. The two approaches - immersion and impulse - can be equally fun, but I find immersive experiences to have more impact on me as a gamer.
I can't wait to experience the full game and be scared the whole way through!