Loads of fun - Lacks challenge and length
This game was tons of fun, but there was no real challenge. If you pick up all the credits you find, you can keep enough strong guns to not have an issue. The only level I had to play more than one time was the one with the moving platforms, and only because I kept screwing up the puzzle part, forgetting to keep people at each place that moved platforms, and so screwed myself over. After the level with infinite enemies, there was no more issue - all my characters were max level after that. Defense with the assault rifle was over powered. Nothing could touch you. Hell, defend was over powered in general, as anything you saw died by the time it was close enough to attack. With the assault rifle, though, ANYTHING that got in melee range instantly was dead.
TL;DR - Add challenge and length. If possible, add extra customization. It was loads of fun though.