The upper body is a bit stiff. If you look at his shirt, it doesn't change form at all. The side of the high shoulder should be straight, the side of the low shoulder should be bent.
In a walk, the upper body and head rocks from side to side to keep the weight centered on the foot that's on the ground. In a real walk this isn't very pronounced like it is when standing still on one foot. Still, it should be indicated and in animation it can't hurt to exaggerate it.
The shoulder line runs counter to that of the hips (high hip is the side of the foot on the ground). In the animation the shoulder line doesn't move either, it stays level. You can clearly see how the angles of the shoulders and hips should have been on the Muybridge photo (every animator should have the Muybridge books).
That's why the animation looks a bit stiff. The upper body doesn't move like it should during a walk. In any natural movement the muscles closest to the center of the body move first (those of the trunk, especially the hips). The rest follows in order of how close they are to the center (shoulder moves before the lower arm, for example).