It's really weird when he brings his leg forward, and then it's only his lower leg that moves. It makes his legs look like they are getting longer. And the way you just reverse the frame for when he starts bringing his leg back... when the foot is on the ground and going back, his leg should be nearly straight, think closer to a bicycle, with a circular motion. His body should move up each time a leg of his is on the ground (directly below him), not just for one of the legs like it is now, it makes it seem like he has a limp. (well, I guess he does go up twice, but on one side it's quicker than the other) Also, it doesn't make sense how his head moves backward for one leg and not the other, it should be the same for each side.
I think there should be more air time, where no feet are on the ground. Also, the timing could use work, less frames for when his feet are on the ground, more when his feet are at the outer extremes, when running the feet move much faster while pushing against the ground, but have to slow down to switch direction at the extremes. Right now it seems very slow because the movement of the feet is so evenly spaced. You've got this basic idea down pretty well with the arms, actually, you just need to translate it down to the legs.
The arms are really short.
Overall this is pretty good, but still problematic. Its got a nice style