First and foremost, this is a great mechanic, seems simple at a first glance but makes for some complex puzzles that require a lot of planning.
The character is nicely animated but overall the graphics could use a bit more work. Adding some sound and music could help too.
I liked the levels (and the thing you did with the boxes) but seems like the difficulty is all over the place, I would reorder some levels to make a steady ramp up.
Overall, the game definitely has a room for improvement but the idea is great and conveyed pretty clearly, nicely done!