I wish that the game had volume control, as opposed to just on/off. I like the music, but it's a little loud, and my computer's volume control is stupid. :(
Everyone's already complaining about the coin system, so instead I'll just say that something needs to be done about the turning around. Chances are, I don't want to attack some air and then turn to face my enemy. Make the arrow keys more sensitive.
In the game, it is said that there are advantages and disadvantages to each side, but it doesn't tell us what they are? Perhaps you could list them on the side selection screen?