Great overall potential!
Those lines that everyone seems to make can get a little annyoing after awhile. Besides that the art is pretty good, simple yet elegant.
The soundtrack is perfect for this game.
A few suggestions on how you can improve this game:
1. More upgrades! Add more weapons and implement boosts (such as health, turrets and special powers fx. Mario's star power.).
2. Smarter enemies. Running around either dodging them or just letting them crash into you one by one gets rather boring after awhile. Make them shoot back! And make them damage you more if they crash into you.
3. I couldn't think of anything else to comment on. However one should always give 3 reasons/suggestions when making a point or writing a review right?
I found it a great game overall, just that it gets rather repetitive after 5-10 mins of gameplay.