ummm... where to start...
well... the gameplay is pretty abysmal. i don't feel like i have much control over the avatar. in fact, the way it controls makes me feel like it's not even really there. it felt like a game that you spent about ten minutes on and weren't even really sure about making in the first place, due to the aforementioned bad controls and the fact that i cannot seem to get a single coin or dodge a single object, and i have absolutely no drive to play this game until i can.
however! i like the concept of a gravity based game instead of a platformer. improve the skins, animation and add a storyline. give me something that gives me a drive to play the game, and make it so that i enjoy it.

Good concept. Execution needs work.

The good:
The concept of jumping between two surfaces which are moving at different speeds and trying to collect coins at the same time.

The bad:
Everything else.

You have something good here. I have played many similar games over the years, but none are quite like this. If you spend more time on the art and add new variations to the gameplay then you will have a good game.

Maybe you can change the character and background. It could be a guy jumping between two skyscrapers or a creature jumping between the bottom and top of a cave. Using boxes seems like like an animator limitation than a design choice. Make something creative.

I also recommend that you change the way death works. I believe you should have one and only one life. If you die then you need to restart. This increases the risk. It forces the player to make tougher decisions, 'Should I try to reach that coin. If I fail...'

Integrate it with the Newgrounds API. Add highscores and achievements. Make people care about the score they get. Then they will care about your game.

Not to say the game is bad, but the game IS full of bugs. The reaction time of the box is slow, and half the time it doesn't react at all. The obstacles will hit the box even if the box misses it by a noticeable distance. The game idea itself isn't all that exciting. Honestly, the two stars are mostly thanks to your excellent choice of music.

the up and down speed is far too slow to play this game to its full potential

This is really hard. But fun! Keep it up. XP