Good graphics, insteresting level design, fun music, simple but very fun gameplay.
Definitely too easy, even with stars... until level 8. Then it gets pretty tricky. I wish levels... say, 3 through 6 had some sort of difficulty curve. (Maybe they do, it's just too subtle).
At first I didn't like the controls, but they turned out to be a nice requirement of skill. I think the player should be able to scroll around the level (maybe with the mouse), for two reasons: the cubes are slow ; and sometimes you want to peek the rest of the level before moving a box or barel.
I noticed I couldn't merge the little cubes when they're vertically aligned while hugging a wall. Was this intentional? If so, you should've still given some feedback - pressing space and nothing happening ... sure, it's a small, insignificant frustration, but it's still frustration.
Also, I must take my hands off the keyboard in order to start the next level. Why?!!? -_-
Definitly a jolly, very engaging game. Good job.