As far as composition goes, I think it would work better with the flower in the very centre, with more emphasis on the light coming from the flower, making it perhaps a more dominant light source.
Also try making larger brushstrokes, with brushes on a higher opacity, this would help make adding detail alot easier.

I agree that it could do with some more detail, perhaps zoom to about 400% and add scratches to the metal on the robot.
Also, the haze around the edges of the piece would work better if they were a bit more subtle.

Great idea though, and you've pulled it off well.
I'd like to see a version of this from a different perspective, perhaps looking headlong on to the character, with the flower at the front, that way you could make it look almost like a movie still.

Hope that helps,
Good work!
Seki