This is an amazing game. Please keep up the hard work. There are a few bugs and some things I would add, so if steven is reading this, he can add them. (I ordered each thing from most relevent to least relevent.)
Bugs:
When I run out of ammo on my pistol, I can't reload any other gun
Double-Jumping is a bit inconsistent, and it doesn't always happen
Additions:
MULTIPLAYER CO-OP AND DEATHMATCH
Death Screen
Invincible Mode (So I can practice and play around with the movement mechanics)
New Guns: SMG, Rocket Launcher, flamethrower, sniper
We should be able to make our own levels.
New Levels: The first demo's level.
If you run out of ammo, I think there should be an ammo pack to refill all your guns
If you are running low on health, maybe you could press a button to unleash a boss monster that will drop a health pack when killed
I think you should be able to pick up dead monsters and throw them at other monsters, and if you kill a puncher-man, it should damage monsters it falls on.
I know this may sound harsh, but this game is truly amazing, and I believe it can change first person shooters