Looks great so far. I love the title screen and the pixel art. Well done!
1. Enemies are lined up. You kill one and you find out down the road you killed several more. If you move up and down and spray your weapon, you can kill all the enemies in the level without advancing. An easy fix is to make it where the bullet doesn't travel past the view of the screen.
2. Player gets stuck ducking against a wall. At fist, I thought the player automatically ducts when shot at. I thought that was cool, but it only happens if you next to a wall. I'm not sure if there is a key to press to let go, but I just press 's' (down) multiple times until he moves away from it. It's troublesome.
3. Sprite overlap issues on level 3. Player flickers when against the wall.
4. There's other things like chopper not attacking, first aid kit not healing and so on, but I understand the game isn't finished.
Other than that, I love the retro feel of the game. I like the celebration the enemies do after they kill you. It makes me want to play again and get him back. It also makes me play harder because I don't want them to celebrate my defeat. Maybe add random celebrations, like having them point and laugh at you? I also like the rain effect on level three.
So far so good. I hope you post a link when it's complete!