Everything from the intro to the menu to the color scheme... It just teems with greatness! The graphics were very, very nicely done despite the fact that the characters were stick figures. The buttons and menu are super cool, although I was annoyed that I left the gun shop every time I bought something. Also on the note of mistakes, you spelled it "Bunty Hunter" on the main map. In fact, there are several grammatical errors throughout the game. Not only that, but when you select a gun for a mission, if you select a different gun, it tells you to choose a weapon when you've chosen one.
The music was especially good. It all fit perfectly to both the situation and the whole style of the game. I wish I could find all of the songs featured in Sift Heads, since the soundtrack is literally the best soundtrack I have ever heard in a Flash game, and better than most console games as well. I only wish you hadn't cut the loops so short. Maybe you could have loaded the entire songs externally...?
The gameplay was fun! The difficulty curve was just right, you never had to wait too long with the cutscenes, and each mission is presented very well. The only thing I didn't like was some weapons didn't target the center of the crosshair, which pissed me off a LOT.
One problem with the entire thing is that the right-click menu is unchanged. I actually managed to play through the mission at the port by hitting Play after it said it was the end of the demo.