Simple but very effective presentation. Substance over graphics, or so they say. The tutorial is helpful and the difficulty rises in a doable manner. I also liked that the game instantly tells you when you are stuck, and how you can restart the game. It's just a very minor thing, but I like it anyway.
As a bonus, the music is calm and collected, which is a good thing, since it doesn't distract from thinking about how to solve the puzzles.