Too short, too easy, but still very fun
Overall the game was very well done; however, I feel as though it was far too short. I managed to beat the game in under a half-hour, and, in combination with the brevity of the game, I also felt it was a bit too easy. I didn't die at all, and enemies are easily avoided. The mini-boss was too easy, and the final boss was also fairly easy, especially with the Machine Gun. [-2 Stars: -1 for Length; -1 for Difficulty]
Also, I am unaware if I am the only one experiencing this problem, but I ended up having to mute the sound. The music was only playing in the right channel, and the sound effects in the left. I tried reloading the page several times, but to no avail. To further comment on the audio, I feel the music could have looped more cleanly; there is a pause at the end of each loop that could be eliminated to produce a better sound. [-1 Star]
But despite the above I, as I said before, found the experience very enjoyable. The look and feel is excellent and, as many have said, is similar to Cave Story - a game which I thoroughly enjoyed. One thing gameplay-wise I especially liked was the ability to stomp on enemies to stun and deal damage. The weapons were also well balanced, although I ended up using the Machine Gun exclusively after I found it. Regenerating ammo and its secondary use as a jetpack made it too good to pass up.
I hope to see a full-version or a sequel sometime in the future. You definitely know what you're doing, and I'm sure whatever you come up with next will be great!