A neat idea and with a bit of polish would make for a really great puzzle game. There's definitely scope to add lots of cool features too (power ups - make one cube go faster/slower, make one cube bigger/smaller, reverse controls, etc).
There are some problems with the game, however. The controls are a bit sluggish, the lack of automatic restart needs addressing (the functionality is there, why do I need to click a button?!), and the graphics could do with a slight polish. That said the simplicity of the graphics does suit the style of game well though, so not too much polish there!
The biggest problem is that the blue boxes don't collide with each other. What this means is that the aim of each level is get the blue boxes together, so that they move as one, at that point it becomes trivial to retrace your steps and get the coins. You could rectify this by either having the cubes bounce of each other, or exploding if they touch outside of the exit gate.
Overall though a solid concept that would be great with a bit more work! Good stuff!