A few things- First, the controls are really bad using a keyboard because I'm forced to use basically everything with the same hand which is extremely uncomfortable... Next, why do I have to hold a key down to use the Compass and Map? A simple toggle would be a lot nicer. A pause button would be helpful but I can see why you didn't put one in given the general theme of falling infinitely with limited control. I would also like to have a little toolbar showing what level my equipment is at currently. Additionally, why can't I climb up my grappling hook? That would also make the hook a lot more useful; As is I don't use it unless absolutely necessary.
All that said, I love the atmosphere and color scheme, and the difficulty is just right for my tastes. (That one torus shaped ship is really cool). Just a little more fleshing out (especially the ending) and this would be perfect.