What it's really about, to me at least, is how people - from individuals, mobs, groups, nations all the way up to the species in general - deals with and adapts to (or not, in some cases) extreme stress and upheaval
You could say that about any disaster movie (I see zombies as more disaster than monster movie, same thing, a vast seemingly unstoppable force comes across our heroes). Its just that the bit where an explanation is squeezed in seems a little bit, dare I say it, too geeky.