Physics
I had problems with the physics of the game. For example, many times a block would react as though I was clicking on it and then "pulling forcefully" when in reality I was just clicking. I would also have the opposite problem when I would want a forceful reaction, I would click and drag and the block would react as if I had just clicked on it. If this is part of the "challenge" it was overly frustrating, and a demo to top it off!