What can I say? This is a fantastic game.
The level design is fantastic - you are deeply lost, until you start paying attention and notice how things interconnect. I love that every piece has some indications to find more and more anomalies - the trails of asteroids, the cube...
Plus, it's got so much content - you never know what you'll find next along the way. It made the game very interesting, and actually a little unnerving. Encapsulated the beauty and sorrow of the unknown perfectly.
And the ending... oh boy.
I could praise it on and on - suffice it to say, it's a great entry.