I really love how you learn how to play the game. You just press start and play till you get stuck at 3. And after messing around you learn you can rearrange the levels. After that you start unlocking new areas, new mechanics, and the eventual end of the game.
All in all this is wonderfully done in game design even with the extremely limited art/audio. I do wish you could rearrange levels, so you can go back, places level vertically and keep falling down. This however means you should slowly add more and new levels.
My main criticism is that rearranged levels should remain permanent between runs, as now that I am relying on my very poor memory and skribbled notes to keep progress.
For its overall Level design I would put this in the top 20 alone.