Fun little game. I enjoyed it!
I know you're not updating the game, so I'll gear my comments more for items to keep in mind for future games. And I know that it's hard to get full basic interfaces for game jam games so take these comments with a grain of salt.
1) ESC or menu button for level select, and level numbers. This game is pretty forgiving since it doesn't restart you all the way back. But for games where you think people are going to be playing for more than 5-10 minutes, it's good to have a level select. Especially since this is more than 20 levels.
2) Most of the levels have a mini tutorial ".gif" at the top, except for level 3. Yes it's easy to figure out what to do, but most of the game is graphically self-explanatory. This is more of a nit-picking item for you to be consistent in how you handle things.
3) Levels 15, 16, and 17 feel like pretty much the same level.
4) The last level (#21 I think) is the type of level that you could take and launch this game into a higher quality game with a full suite of increasing difficulty levels. I think the spanning idea is really neat, and you could take the idea of spanning, breaking, adding, spanning, breaking in a single level and really run away with the puzzles for this game, by making the player touch 2+ exit portals simultaneously on the same or different platforms.
Again I know you're not updating this version of the game. But I think with the concepts you introduced and some hardcore level planning, and polishing up of the UI and some more music, you could do a sequel or bigger version of this game that is mobile app store $ quality.
Good Luck!