Hatsune Miku ver.SYNC

I'm with everyone else on some of the line weight issues, although it looks like you're punching them up a bit. The head of the guitar is too close in color with her hair, I think; it doesn't pop enough for what I think you're going for, and in this case, since the lines flow with her hair so well, it does kind of disappear.

Another thing that might help is to deepen the colors in the background gradient. The image currently has a nice sort of pastel feel to it, but I'm not sure that's the best approach for this. Not quite enough contrast, I think.

I mean, really, this is pretty awesome, so any legitimate critiques are pretty minor.