Love the art, the spinning coin looks really cool for a 4x4 sprite! I stopped after trying the fourth mine for about 10 minutes. The level design is good, though a bit on the sadistic side :P
Having some dust effect when landing or jumping would help a bit with all those long jumps that require you to jump after you run off the platform. Also, I think the spikes don't look menacing enough. When I first started playing, I kept accidentally running into spikes because they didn't look like they were an obstacle.