The difference between World War II and a post-apocalyptic setting is that, in World War II, you're more or less restricted to, well, World War II. On the other hand, the post-apocalypse can take any number of forms, and is restricted only by the imagination. The problem these days is that there doesn't seem to be any room in the imaginations of developers for anything other than dirt brown-coloured shootery settings, with a few notable exceptions. Take, for instance, Enslaved: Odyssey to the West. It's set in a post-apocalyptic landscape but, given only a cursory glance at a few screen shots, and the current perception of what defines a post-apocalyptic world, you'd never know it. While not redefining the context, it at least shows a modicum of imagination.
I love the idea of the post-apocalypse in general; what I hate is that they all look exactly the same, when there's absolutely no need. We've already seen this wasteland before, in 30 or so other games; it's just being lazy.