I played through the game and I can say I had fun :) It's very Cave Story-ish.
I think you nailed the difficulty. I died maybe 5-6 times(most times to the final boss). It never got frustrating or too easy for me. I also like how fast paced the game is.
I did get tired of pressing X repeatedly, so I think it would be better if the gun was automatic (Hold X and the gun shoots until you stop pressing). Really that's the only gameplay problem that annoyed me. Other than that, it's all small things. For example I found the visuals to be a bit glitchy. There's some tiles that flicker and a lot of visible seams in the terrain.
The dialogs are a bit confusing and sometimes funny. I am guessing english is not your main language. I think I got the gist of the story though, and I liked how short the dialogs are. In most games I avoid talking to NPCs, butI didn't mind talking to people in this one. I especially liked the "Be near trash... Become trash..." robot; Not sure exactly why, but I think it's a very nice quote :p
Overall I'd say you did a good job :)