like pypebros said, the animation needs weight. right now it feels like he's running in outer space. the movement of the legs shouldn't be steady throughout the animation, there should be points where the animation 'snaps' a bit, like when he pushes off of the ground with a foot. the movement is right, but perhaps some of the in-between frames should be dropped. also seconding pypebros second crit, for a natural looking animation it's often best to sketch the full body out, since the full body is involved in running, and not just the legs. bobbing might be enough for a slow walk, but for a running animation I expect a bit more movement in the arms and torso.