This game is decent for a demo. But I have a lot of suggestions.

-the pixel art is 8 bit but the music uses sega genesis sounding instruments. Unless you plan to redo the art to 16 bit sega genesis standards later then it's unfitting.
-on my xbox gamepad, jump is 'B' and shoot is 'Y'. It should be 'A' and 'X' respectively. Or just let players remap the controls.
-the first enemy is too close to the door from the charge switch room, so if you keep running forward after exiting it you will run into the enemy with no warning. 
-full screen has black borders instead of stretching to the user's monitor aspect ratio.