It's the aesthetic, the sound, the tunes, the clever level design, the leaderboards, the neverending tower, the writing even. I never thought I'd be praising the writing in a level design. It's smart and it's funny.
Also, the learning curve is really awesome: you literally learn how to play the game in 2 seconds, but the physics and the way the difficulty is gently increased from level to level, ensure that you'll be mastering the intricacies of the system for ages. The sense of accomplishment when finishing a harder level is priceless.