the end-of-level fanfare and option popups (next level/etc.) are immersion breaking. it should just flow into the next level seamlessly.
didn't like having secrets in levels that you can't really find without pressing against random walls.
this game would have mega-benefited from some light story throughout the levels or every few levels.
there is trash then there are games like this that have enough to keep you interested but are lacking some key pieces (or possessing some key annoyances) that keep it from being great. this one is right there, imo.
oh, and finally it has some bug where your movement keys will get unresponsive. i died in the middle of a level because the down key stopped working. (obviously i hadn't lost focus in the flash window because the left and right keys still worked)