So I just got Proteus in Humble Bundle 8 (god I love Humble Bundle), and went into it with basically no knowledge of what it was.
That was fucking awesome. My first playthrough lasted 49 minutes, and I consider it time extremely well spent.
I really recommend this, it's very interesting and totally worth an hour of your time. And you should buy the Humble Bundle anyway, because it's awesome.
Don't want to spoil it much, but here's the general gist of the gameplay:
You arrive on a randomly generated island, and walk around it. The graphics are lo-fi but the landscape is pretty nice to look at. The soundtrack seems to go along with the gameplay somehow. Scattered around the island are all kinds of little creatures, plants, and structures, and they all make different noises. The magic of this game is that they somehow managed to make this awesome and not obnoxious. The game's very immersive, you'll be pretty much zoned into the screen unable to do anything. Something about this game's design really taps into you, and you'll feel excitement when you crest a mountain, fear when you approach some ancient statues at night, sadness when you realize the summer dragonflies have died off in the fall. It's really weird, and really excellent at the same time.