Edited.
The compact levels and snappy restart means you can iterate your play at a pleasant speed. I like how it introduces new concepts in classic fashion.
The way you can stretch and automatically bounce, around yourself without lost speed is an example of polish that I appreciate.
The seconds of immunity frames at the start, and the way you can clear surprisingly complex levels in a single stretch and have that moment of feeling like a genius is just perfect, and again speaks to a highly refined game that is quite rewarding to progress through.