I loved this game! Love the graphics, the physics feel, the whole thing. Feels very polished, and would be a memorable 90's console game.
Now some complaints!
The rotation kind of messes with my eyesight, I kind of have to unfocus my sight and not look at any moving part. Not sure how to fix that, but just thought I might tell you this could be an issue with more people. It's worst when I look at the middle ground ball when it's rotating, or try to follow any rotating objects with my sight. But it's fine when the character is stationary (this is an issue that i've noticed with myself when playing other games which feature full screen rotation. I can play FPS games fine though)
It's very unforgiving on placing spring and platform errors, if you make a mistake you have to restart the level. Maybe give the option to turn the platform or spring back into a bonus ball, or just absorb them when pressing their "place" button? I got stuck on that level where the exit is in the air since I, repeatedly, couldn't place the platform in the right place (which also happened due to the problem I mention next).
I could never figure out where the platform was going to appear. Maybe pressing "Q" makes a a little preview box on where the platform will appear, and then mouse click places it. Could even allow the user to "aim" where the platform would be placed, within a radius around the character.
I constantly forgot which key does what. Maybe keep a small drawing on the top corner on what key does what - as in "Q - flip, E - platform, F - spring"? Or reduce the number of keys used, by using an "inventory" key, that activates an inventory (Minecraft), or flips between the available actions, and another to "use" the action selected (Comix Zone).
The music is ok but repetitive, I shut it off, personal taste absolutely
I liked that I could kill the enemies from afar, without having much faith that I'd hit them, but ending up hitting anyway, that gave me as a player a "nice aiming skillz, I rule" feel, that's good
Good luck with finishing the game!