Biggest problem i see with that one is the perspective (or lack there of) on the feet. and allot of it for that matter. but here's a tip:
Always remember where u want the eye to view the image from. i usually go with about the waist or just about it. When you look a t a curve straight on, it almost appears as a flat line ______ as u move down from the point of view, curves curve downward, and as you move up they curve upward.
Lets have a quick look at my good friend Stick Figure Wallace
http://img379.imageshack.us/img379/9484 ... ivelf5.png[/img]
So basically according to the ideas of perspective drawing, In your above picture, the flattest line is at the feet, there fore that is your focal point (it shouldn't be but that is where it is) and therefore, the vast majority of lines that curve downward from there should be curving up (not all but allot) like the buckles at the knee, the Cuffs around his arm bands (which could also be ur focal point as they too are flat, and actually probably should be) the curve of his collar.
If the focal point is form the arm bands, then the legs are actually not too bad but the feet are still a problem, and the neck line of his shirt still probably shouldn't be dipping down.
You have some good designs, and your work isn't hopeless by any stretch of the imagination. Learning and applying perspective to your work (or at least effectively faking perspective because believe me i don't go back and check every single line in my work for accurate and perfect 3d perspective..) will really help you leaps and bounds.