-You may want to put weapon upgrades on your weapons too rather than just upgrading your feets' weapons.
-Could have more variety of weapons(flamethrower, grenade launcher, etc)
-Needs a pause button.
-Gets repetitive after a while.
-The controls would sometimes get stuck or something(may just be because of my low computer).
-BG music gets annoying fast(may not just be my taste).
Overall not bad, a little rough but i enjoyed playing it. btw i'm at lvl 16 and it kicked my ass. BIG TIME!