The atmosphere, the atmosphere's to die for.
That feeling of wonder and mystery, the way the water puts you more or less on top of everything and yet it manages to make you feel small. I can nearly feel the cold air and uncomfortable wetness through my screen.
It's all coming together really nicely to chill you down to the bones and yet make you feel nice and safe in the warmth of your own home as you watch it happen on your screen. It's something I aspire to achieve with my own games.