Nice concept!
The coding and controls could use a little work though. The jumping is kinda sketchy, if you know what I mean. It's the same problem many flash pltformers have. The graphics could have been better, like giving the coin some detail and depth. It's harder, but not impossible. I could show you how if you needed. Next time add the option to set your own controls, too.
Great game, I'll have to pla ythis more later. Losing the shell is great btw, perfect way to represent "little mario".