To be honest, your line art in this is very solid and that's not always easy to do
This picture would be even better if you made it into a vector image, added more detailed lines in the pants and hair. Also, since this picture mainly has nice solid colors, good shading would compliment your lines well. Here's a tutorial from Omar Dogan that helped me ALOT. If you take the time to follow the steps, you'll see improvement:
http://omar-dogan.deviantart.com/art/Colouring-Tutorial-72826280
BTW, I added a comment to your deviant art page