This could be a quite enjoyable game, however, there are missing some parts I'd personally like to know in any game.
First off, a decent taste for style, or at least some overhauled graphics. The background is okay, the mix of colors, however, hurts after a while. Not just that it is too randomly colorful, but all the stuff appears plain and the supremely bright colors don't hide that fact. Nut just that, but the screen further on seems to be quite ... I wouldn't say empty, but it really should be more detailed (not meaning you should stuff even more colorful stuff on the screen, but rather change what already is there).
Mouseover detection - especially for the play button - should be improved.
The game is missing a story, and yet we have that strange star-eyed guy right there. Explanations, however simple they might be, should be there at least (dunno what happens when you beat the game, never got there).
Next: The controls are awful. Honestly, they don't really give you the feeling of controlling a character. Would have felt way better if one would accelerate the character to a maximum velocity over a small amount of time, and set his jump heigh according to how long the player presses the "UP"-key.
Last of all I'd also mention - like some before me - that the collision detection also is very inaccurate and spoils the fun a lot, since one can collide with the stars hitbox, instead of the char itself. You should read more about bitmap collision detection and learn about the pixel-perfect collision. Don't worry, it's quite easy once you understand.
Other than that:
Stay with us, and keep the Flash rolling!
I'm sure you'll get the hang soon, so just keep on practicing.