I don't draw anymore, but I did take a class a few years ago to fill an elective and my primary criticism would be anatomy. I made so much progress drawing random poses over and over again. My teacher used real people and gave us about 45 seconds to do an outline as fast as we could (joints, perspective, etc) then another 2 minutes to make the actual body. At the time I thought it was stupid, tedious, and a waste of time, but it made me so much better at making actual people in terms of anatomy. I wish I had an example to show of what 1 year did for me (trashed all my drawings). Since you might not have the same luxury of people to pose for you I'd suggest www.quickposes.com I think if you do just 10-20 minutes of these a day with 2-3 minutes per pose it'll greatly improve your work just over a few weeks.

I also really agree with what someone else commented, make your art much larger. I was taught to fill my page, allow room for details and shading.

If you work hard, practice daily, learn from other people, and take criticism you'll get to where you want to be in skill.