I'd like to see this expanded on.
A couple things: I keep instinctively hitting R to reload and keep restarting the game. I suggest changing that.
Also, the only armor pickup is in the last room with enemies. I played through all difficulties and only got the armor after everyone was dead.
A few other FPS conventions could be used like scrolling to select weapons.
EDIT: also realized that the main menu is titled "Options" and clicking options takes you to credits.
Collision detection for the enemies would be good too, so you don't run right through them and have them attacking you from the back.
The melee attack was useful, but I could see most players not realizing they even have it.