I like this a lot. Theres alot of energy and character in the upper body, the hand, the face, the arms....really nice stuff. Your moving the shapes of the body around really well. However you loose a lot of that energy in the lower body, mainly b/c neither of your lower limbs are following the flow of the spine(the action line), your legs are taking off sharply and hiding behind your figure,therefor causing you to lose out on a lot of the energy in the silouhette. And the left leg looks too contourted,or visually...it's a strange angle to pull off. This is really nice work with a lot of movement...I'll do a quick over doodle to show you what the hell I'm talking about...

Leaving your upper body alone, I just lined your right thigh up with the curve of the action line(established by the spine). Then I tried a handful of different takes on the left leg, used as a counterweight. I like the black version best, only it breaks the rule of not having both the limbs being bent in the same postion, but the character outline works well enough to make up for it. Then I did one with the upper body infront of the punch,like your orginal version. Then one with his leg kicked out at us. Sorry these suck, I only have a mouse and the ball sticks. Anyways....hope this helps. Really cool sketch and your version does have a shit load of energy in it,a lot more than my sketches over it...only its tough with that left leg disapearing.
~nick