Looks pretty good to me -- I don't mind the feet sliding while walking/running but some might pick at that. IMO, that's about the same quality as a lot of Kongregate-type games I've played. I think I see is a bit of jerking in the horizontal as you go through the stride sequence, maybe a matter of inconsistent reference points for frames of different width. Subtle though, could be a fraps issue. Slow motion might facilitate a better review.
I think it might be part of the sprite. I might have to look into the jerkiness. I do see it right now. It could be an interval difference. Thanks!