Some feedback: the tutorial room doesn't work; you just get thrown in an empty room with a non-functional door until you exit via the pause menu. Also, there should be subtitles during the intro cutscene, and I'd like an anti-aliasing option if possible. By the way, is it just me, or does the camera move at a lower framerate than what's set in the options? The first boss arena especially felt low-framerate no matter how much I lowered the settings, and when the boss killed me, I wasn't able to pause the game anymore until I beat the boss.
As for gameplay, there's too much forward momentum, which makes precise platforming unnecessarily difficult (especially this early in the game). 3D Sonic gets away with it because of the homing attack (which instantly kills momentum upon hitting an enemy), and 2D Sonic gets away with it due to there being fewer ways to miss a platform or lose track of yourself in 2D space. Maybe you could add a ground-pound move to get around this issue? Also, the jetpack and forward-dash should switch buttons; there were several times where I'd go to jump, only to dash off the platform instead. Swapping the buttons would fix this while also letting the X button do something while on the ground.
On a different subject, it would be beneficial if similar-looking objects that act differently from each other get recolored (such as ordinary wood vs. collapsing wood or normal sand vs. death sand). Plus, the death sand should kill the player faster than it does. Also, I'd appreciate if pushing the red button in the sand level had the camera cut to what it changed, like the Zelda games do.
Lastly, how do I attack/kill the tower turrets in the sand level without getting hit myself?