I generally really enjoyed it, for an early demo its overall pretty good. I liked the idea with the hub, however i felt that i had no incentive to skip a level, and there it felt a bit (tiny bit) tideous to run through the hub level every time.
I played on my macbook, and the window was really big, when in windowed mode (i suspect that you have the standard resolution set to something that fits on your monitor), so big that i couldn't see the fullscreen button that you had implemented in the main menu. I did get it to fit my screen, but a resizeable window would be very nice.
But overall great job with this demo, i also really liked the graphics and the feel of the character.