Why does my armature's bone rotate 360°, but the mesh only rotates 180°? I've got a cube-shaped mesh and an armature. Both are aligned, and weight-painted properly. What's happening is that when I rotate the corresponding bone for the armature, the actual mesh rotates half the amount of the bone until I reach 180 degrees (of bone rotation), when the mesh jumps to the other side and continues its half-thing.

In short, the mesh is only rotating along the top 180 degrees of its full rotation cycle while the bone is able to move at its full 360 degree rotation. It's really acting as if I put an IK rotation constraint, but that's not the case. It's a full three bones of separation from the nearest IK bone, which is not connected to it in any way.
Your head also inherits roatation from the torso bone. Look at the vertex groups: The head's vertices are members of both groups, 'Head' and 'Torso'. Thus the rotation of the head and the torso bones are combined by a weighted average. The weights of both groups are 1, so this results in equal influence of both bones. The torso-bone does not rotate, so the total rotation is halved.
Just remove the 'Torso' vertex group from the head and it rotates as intended.