Fun game
I liked the gameplay, it worked very well. The graphics did their job well too, stylised and clear.
Could have used a few extra music tracks. They were good but overused imho.
I found the early maps nice, but later on they felt repetitive and quite long, especially since we didnt get any new ennemy to fight, it was just more of the same (but stronger).
I expected something like a "pacifist" award for each map, like not killing anyone (or no more than 5 ennemies for advanced maps for example). Unless im mistaken i only saw it for the first map.
I just really disliked the ending wich was a huge letdown after all we did to get there.
The rocket laucher was my favorite weapon, despite the low ROF. THe massive damage, bullet absober and flak effect just made it really, really good. I barely used the other ones. I only used rebound as an "early" secret detector.
Your gameplay formula worked quite nicely, with the different items opening new access to a few secrets in earlier sections.. ( but they were kind of few and far between imho) because of that i feel that you could make a spin-off of that game, but within a metroid-vania map/progression/gameplay.