First of all, I must say that I have fallen in love with this game. It's fun, it's simple, and it's charming. However, there are just a couple things that I would like to see fixed.
Firstly, the game feels like it gets a bit boring once you unlock everything. The weapons all do the same thing, and unlocking all the characters isn't difficult. Perhaps giving different effects to the weapons, or allowing the player to upgrade their unlocks with extra coins, could be implemented. None of this is really necessary, but it may add more replayability.
Also, there are a few glitches or other gameplay quirks that could be fixed. Most noticeably, the game starts to slow down after a while. Also, pressing the jump button twice in rapid succession has some weird effects. Sometimes it creates a double jump effect, and sometimes it doesn't. And finally, a TNT space directly before an enemy with 2-4 hearts is effectively instant death, as far as I can tell.
Overall, it's a great game, but it isn't perfect. I'm guessing there will be improvements some day, though. Even in spite of its flaws, I'll continue to play this lovely little game.