Here's some constructive criticism for ya

The big list of cons/suggestions:
-Glitchy collisions.
-Bad graphics.
-Getting around the level select is annoying. Make one press of the arrow keys move you to the next level at a faster speed.
-Waiting 3 seconds to start off the level after dying is annoying. Make the timer go twice as fast. Also, you should add some sort of transition effect for the timer, like a clock or making the numbers fly across the screen.
-Some of the levels are too hard.
-There should be some sort of preview at the beginning where you scroll from top to bottom. It should only happen when you select a level, not after you die, and it shouldn't be too slow. Also make sure that it's skippable after you see it once.
-Realistic string physics for the balloon. It may be hard to make, but I love me some polish.
-More sounds. It seems quiet.
-A particle effect when you collect those balloon things. Shouldn't they be presents? It fits the story better.
-I haven't beat the game yet, but if there isn't a final boss, make one.
-Make the medal times easier to get. When I play, I naturally am not pressing up most of the time. Don't make it super easy, but shave a couple of seconds off the silver and bronze.
-It said I unlocked something, but where is it?
-A level unlock animation, where maybe the background itself changing, like a castle lowering the moat. Also key unlock particles.
-Make the overheat system in the desert level look cooler. Maybe a bar, and when you get close the screen flashes red and stuff. Also make a different death sound and make the balloon catch on fire.
-More levels with more things.
-Coins that give you points.
-More cutscenes.

This game has potential, but it's not there yet. This is a hefty list, but I am helping you. This is mostly a list of suggestions rather than cons. (Okay, maybe it is pretty con-y.) Not to nag, but I'd prefer if you gave me the source since I have a vision and I don't want any of you guys besmirching it. I might add even more.

PS: I tried putting the list in the cons section and it deleted most of it, making me start over. Ugh, make sure not to do that.