Well, a couple of gripes
First off, great game, good graphics, nice interface, etc. Now, on to the bad. When you die the enemies dont respawn, which is an OK thing, except that it makes it impossible to double jump, sprint, etc, making the task of getting back to where you already were a very difficult one indeed.
Another small issue I have is that there are never check points near bosses. This is exaserbated by the issue mentioned above.
My last gripe is the purple level with all the floating blocks. That level was the most annoying, frustrating, super-mario-crap level in the history of gaming. I got sick of this game after ten minutes of repetatively dying, respawning, and dying again. When I finally was getting the hang of things, I killed someone and started sprinting quite by accident, effectively ending that run and almost ending the life of my laptop. I actually simply closed the game and decided to be done with it, because no ending could possibly be worth spending an entire day repeating the same level.
Other than that, fantastic job, man!