very nice job. I love the Metroid theme layout, the visuals, the different abilities of each character, and just the over all feel.
The only points of improvement I have are that your difficultly curve is pretty much a vertical wall requiring pin point accuracy on level 1, so just build in some tolerance for mistakes and you should be good. This is more of a suggestion, but I would also advise tweaking the hang time and speed of the other cubes to really give them a distinct personality.
Otherwise well done, would recommend!