I dunno, perpetual invincibility? It made it just a tad bit too easy. The fact that I hardly ever even needed to use my shuriken was something somewhat short of annoying; all I needed to do was fall on or crash into an enemy and WHAMMO! -- he's dead. It also got kind of crazy whenever I was being shot at because the bullets coming in contact with me seemed to have the same effect as when I step onto one of those speed-up pads. Though I guess I kind of liked it that way (the craziness in the physics of this game made it different and fun in it's own way.) One last thing to mention is that the sprite looks kind of... stiff, I guess, but not too noticibly so. Still, in all it's simplicity, the game was actually all in all pretty cool. I particularly liked the parts where you would get chased by a helicopter.
So I'll rate your game 3 (out of 5, as you requested.) Your game certainly could have been better... but it still was fun to play. Heck, it was so different, interesting and short, I'd like to try it again -- and I just might. Aw, scratch that. I probably will play it again. Good job!