Weighting in Line and other bad puns
I like your line work - very clean and I can easily tell the direction of the light source even if there's not a lot of actual shadow. That said, I agree with some of the others about variation in line weight.
This one, for example, could use thinner lines for the fabric folds on the stomach. That would make the chest pop a little more and add more depth.
I've been using a cartridge-fed brush pen since I can take it to work and ink over lunch. I'm still learning it, but I've been happy with the line variations it gives.