Great visuals, great music, great level design, perfect use of theme, fun gameplay. The only problem I have with this game is that one level (you know which one I'm talking about) and it's not a big problem at all.
Also watching the bossfight beforehand and knowing how to beat him definitely made it a bit easier, so it's hard to say how I would've fared otherwise. But you could definitely indicate that the boss is vulnerable in a better way.