I would suggest working on giving a little more meat to the arms and legs, but then again it all comes back down to proportions and anatomy. I realize anatomy is incredibly difficult, especially when you change it stylistically, so take a look at some manga artists and how they go about drawing, some of the people on here are great artists to use for reference. In the end though, just keep at it! As long as you keep at it, as well as doing lots of studies, you'll keep improving. Best of luck, from one novice to another!