Great game. Though I was getting a bit bored by zone 13, ended up dying that round to friendly fire, I was shooting infinite bouncing rockets around as I ran. Actually worked for awhile, because I'd get more health when enemies died. Also ended up using the one bullet gun as a gun for awhile. Though that created some lag so I just went back to the shotgun.
The number one thing I'd suggest changing is the sound it plays when you pick things up. My first thought hearing it was that I couldn't pick up what I'd tried to. A "you can't do that," sound.
Otherwise it would just benefit from more of everything it already has. Great work.