"The sheer lack of variety across the missions, in terms of enemy and level design, is the defining thing that sinks the campaign mode. After half a dozen missions of doing the same thing, the magic wears off. As you progress you'll discover that the story is pretty basic; there's no real explanation for the disappearances or any real motive behind it all. Plus, the boss battles themselves are pretty pathetic. Most are dispatched in the same manner, which vastly cheapens the experience of taking them down."