Is nae bad, e'en fer a demo. Personally, I like the super-retro Gameboy-green aesthetic (probably because Super Mario Land was one of the first video games I ever played), and the music is decent. That said, I would suggest adding some sort of knock-back effect upon contact with enemies, since being able to walk right through them feels a bit awkward. Also, since the enemies only ever come from the direction you're going, shooting straight up is much less useful than shooting at an angle, which in this demo can only be done while moving; I would suggest making the up and down directions fire at 45-degree angles, so that they can be used to target enemies coming from the right side of the screen. Furthermore, you may want to implement a checkpoint feature that activates automatically after every sub-boss (though you could also omit this if you want the game to feel really Nintendo Hard--it all depends on what feel you're going for). Lastly, if you give the various guns more stopping power when killing enemies, it could counterbalance making the enemies stronger and/or reducing the maximum vitality which, combined with knockback, would make the enemies much more dangerous (so, both the player and the enemies would be stronger, which would make the game feel more intense). Sorry I got so long-winded, but this demo feels like it has potential, and I really hope to see it made into a solid final product.