Think I've found a minor bug - sometimes when you jump on enemies underneath blocks whilst holding the up and (left or right) arrow keys, you seem to bounce on thin air for a while and get stuck. Just thought it's worth a mention in case you haven't noticed yet :-)

Odd, I'm sure that wasn't working for me before! Either way, nice start on this - perhaps the movement could be improved a bit? An animation of mario would make it look a lot smoother, and checking for collisions on the side of blocks as well as the top should prevent mario entering in the middle of them. Animated stars would also look cool! ;-)

Possible (minor) bug - on the third level, if you crash into the wall on the left hand side about half way up, the fuel meter explodes after the rocket does!

It'd be good if you could allow going diagonally as well (i.e. by holding the up and right keys down at once.) Nice simple game idea though :-)