Your sketches look pretty good ... are you drawing these line by line? The weight seems a little off - the lower half of the body has perfect squash to the weight, but the shoulder shapes seem to cancel out the up and down action on the torso. (I've been staring at this for 15 minutes and I still can't figure out how the body doesn't look like it stretches, on the contact, but the figure does.)
It's likely an issue of how little time a had to work on it, because it had been drawn up, scanned in, cropped in photoshop and complied into an animated in flash. As such the actual way it animates isn't very well thought out at all, I did still tried to make it look good in the minimal time given but I know there isn't nearly enough bounce to the step.