This is definitely excellent and this isn't a "I'm going to vote greenlight", no, I did greenlight because the game is fun, intuitive, and a challenge generous to my ready-to-learn mind.
The presentation for abilities is fantastic, learning what each one can do as they come. Hidden rooms, pushing boxes, and the rotations become simple as you demonstrate them by allowing the environment to naturally expose us to them as we fall or rise in the dungeon levels.
Music was repetitive, but had a nice pace and feel to it. The ability to turn on and off SFX and music is a helpful design for players as well as the fullscreen change and rebindable keys. Also the two player experience is a fun concept as well. My impression on the co-op system is that it is more a competition than a regular friendly experience. It seems easier to solve puzzles on your own (I tried playing both) and the counter at the bottom makes me feel I need to race to collect more. It is obvious though that this friendly competition for "who gets the most" comes in handy because the combined count unlocks rooms ahead. It is competition that benefits rather than hinders. (Its like shooting your buddy in the butt except it heals his face.)
Flightless has a gorgeous look as well that fits the Nitrome game style; and it is awesome that you guys are sticking so well to a style that not only works for you, but is recognizable as yours.