I see it what you're going for...

...and this is why I think you fall short.

This style of drawing really only works if it's "perfect" and by perfect I mean vector line work. All in all it's just not clean enough. If you really really don't want to work with the bezier pen tool (or in Illustrator) then I suggest at least going into your "Brush Options" and turning your space down to "0" that would account for alot of the "squiggliness" of the lines.

The other aspect of this is that if you're not going to use line work (and who wants to really) then you've got to have a stronger understanding of color theory. I know you used several different colors there but see how they all "blend" together especially in the top portion of the piece. Use high contrast to define your shapes rather than just slightly different colors.

If you convert your picture to greyscale you should still have very defined characters and objects in the scene.

So take those two things, splash at least a HINT of texture/lighting into the scene (even if that texture is glossy) and you're starting to move in the right direction.

Don't get discouraged! Just trying to help! Keep your chin up!