Thank you for the great feedback!

Machaira wrote:
It looks like the feet are coming up to the butt. I don't know that anyone runs like that. Other than that looks pretty cool.
I know. I'm trying to make it exaggerated a bit (not too over-the-top, but certainly with a tinge of "cartoon"). Too much?
Codehead wrote:
However, it looks like the lower leg is swung forward by the upper, that gives it a bit of a rag doll feel. I fact it reminds me of Woody from Toy Story.

Hehe, I noticed that, too.

I'm not sure that's a bad thing (it gives a little bit of "looseness" to the animation), but I'll probably tone it down.
Codehead wrote:
I think if the lower leg stayed bent for longer while the knee drives through, it would convey more of a sprinting/power feel.
weezl wrote:
It kind of looks like he's ice skating, the feet almost stay in the same line of movement in both directions, almost like a pendulum. Try to raise the knee higher when swinging the leg forward.
Yeah, I think this is the primary culprit with the "wimpy" look that's been nagging me. Not the run itself, but the mood. One person told me he looked too light on his feet, and I think adding some spring (via a pronounced push-off, along with a shin with more tuck) would alleviate this "ice skating" effect. (Also some "camera" movement in the image itself would probably help with the illusion of motion.)
Again, thanks for the comments, much appreciated!

I'll update you when I have tweaked the image.