Fascinating game! It's wonderful how it does so much with so little. When I rate any game, I rely on three major aspects: the extent to which it utilizes its gameplay mechanics, how this mechanics interact with the story and artistic style to produce an integral aesthetic experience of a game, and how fun and interesting this experience is to me. This game, in my opinion, manages to succeed at each of this aspects: the very few of its gaming mechanics are quite simple and yet allow for great variety of challenging puzzles; game's minimalistic story and visual style are set just on pair with it's simple mechanics; and finally, the overall experience I get was very satisfying with interesting challenges to solve and some funny comments to read between levels.
I also liked the 'transparent' background. Never saw this effect here before, so it felt refreshing and complimented game's overall visual simplicity and restraint even further.