I think what VictorR is trying to say is the lines look a bit rough. It's not so much that they're rough, just that there needs to maybe be a bit more depth so it's more noticeable. try adding a few more colors or maybe a highlight like on the more defined areas.
Here's an edit I made to show you what I mean.
I'm not saying thats exactly how you should have it, but a brighter color may bring out some definition that may otherwise go unnoticed.
Oh Btw, I love the cell shaded look, works well with this piece.