Too subtle to notice -- try adding a bit more of a shift downwards with the point of the hair on the frame after her head lifts back up. It needs to have a perceptible 'bounce' to the hair since the head pops up and down from the step and gravity is still acting on the hair and the body when it does.
Also, on the frame where she's in the air completely, it should be considered the "contact" pose for the frame, which means that at least the heel of her front leg should be touching the ground to give a sense of weight. The next frames should be her weight falling on that front leg, and it passing beneath her body while the opposite leg begins to come forward.
After all, a walk (or a run) is simply a series of controlled *falls* and she doesn't look like she's falling at all in the animation, thus the comment from r1k about the foot coming closer to the contact position.
As far as the arm goes, she does need her silhouette varied a little bit -- even if you lift the left arm higher than the right by a few pixels or bend the elbow on it a bit more, it needs a more readable silhouette to distinguish which arm is coming forward just by taking in the shape of the image, before your brain ever has a chance to process the colors. Otherwise, your animation's readability is pretty poor.