Amazing! This is truly the best thing I've played in a long time. Well done, especially on the boss design. The graphics and music have a nice feel, personally it reminded me of the old megaman games. I particularly like the way you teach the player how to play your game. It's fairly plain and intuitive at this point, but the way you introduce the falling platforms is done masterfully, again reminiscent of megaman's glory days.
While I really, really enjoyed the demo, I have to confess there are several design choices I disagree with. These are, however, my personal opninions, so feel free to disregard them, should you see fit to do so.
First of all, the weapons. While it is good to encourage the player to keep playing at the beginning of a game by holding their hand a little, in my point of view the weapons are totally overpowered. As soon as I realised I could hold down the fire button, enemies became a minor inconvenience. Why not make the player lose their upgrades if they get hit, la Gunman Clive?
Secondly, the health meter. I know numbers add more variation and balance to the health system, but it would be far more understandable if you simply used a health bar in the HUD. Intrusion 2 had a balanced health system, without overflowing the screen with confusing data.
Thirdly, and this is very personal, you might want to zoom out the screen a little. I assume you haven't done this already as an homage to the classic Game Boy titles, but frankly, it's just needlessly annoying.
Finally, the bosses. Yes, their design is awesome, but I feel like the first one had too much health and became more of a bore than an actual struggle. The third and fourth ones, however, were defeated far too easily by my intensely overpowered gun and heaps of health points. I adore how the fourth boss uses mechanics from the first one, but I killed him as soon as he entered the screen for the second time (I had the flamethrower at that time). The pong robot was very well balanced, on the other hand, and though he never really posed a serious threat to me, he was challenging but fair, and altogether simply fun to fight. I noticed that the ball started moving faster as I drained more of his health, and I feel this mechanic should be included in the other bossfights as well (come to think of it, maybe it was, but I just defeated them too quickly).
To sum things up: your game is awesome and so are you, but you need to balance out the difficulty if you really want to honour Ye Olde Platformers. Other than that, amazing job, old sport :)