Some suggestions:
a) Offer an XNA package, but also release the source under an
OSI-certified license
b) Alter the graphics to indicate the number of blocks per stack (for example, two squares on the yellow, three squares on the green...)
c) Prevent advancing to the next level until the current level is complete
d) Add more intermediate levels and make the advanced/impossible menus not crash the game
e) Make the menu text much smaller