First and foremost, love the game! I loved figuring out how to play, and how to communicate with my partner. [spoiler]
I also really loved the end, where I was able to communicate with my partner, and tell them where I was. [/spoiler]
The only thing I didn't like, which I wish was changed, was the transition between 'levels'. Instead of just cutting to black, which the first few times made me think the game crashed or something, maybe have the players pass through doorways to imply the sense of transition, and maybe also have a little loading symbol in the corner just to give the player some feedback.
I realize that this is an alpha build, so it makes sense that it's short at the moment. Do you have a goal point for how long you want the game to be? Also, are you planning on allowing players to make their own levels? With a game like this, replay value will be a big selling point, and since during the first play through you're going to understand how to play you can't rely on that gratification of the discovery to keep people going. So, unless you're going to secretly roll out new game mechanics for people to figure out, new levels will be a must.
Other than PC obviously, are you planning/aspiring on taking this game to any other platforms?
What sort of price point are you guys thinking of? While not saying this is what you have to do, my thought with this sort of game is that it's release price should be low (SUPERNEWGAMESALE!) to get a large player base, since this game relies on having other random people searching for a partner. Steam has really shown that those sorts of sales really work, and once that initial user base is established, the price could come back up to the 'regular' price, as the user base spreads the word of the game about.
Really looking forward to the progression of this game.