Quite good for a first game in unity, a few things I would like to suggest, work on the camera, its very jumpy, I know I've seen some scripts that can smooth it out, add a aiming reticule, it is really hard to guess where the snowball will actually end up, and then work on polishing the graphics, but I'm pretty sure you were planning on doing that anyway.
I really love the game, since this was for a 9 hour compition, i know u might not want to update it, but i would suggest making a sequel with upgrades and stuff, its a really fun game. BTW as i post this i hold the #1 high score :D
and Merry Christmas