At The American Scene, Noah Millman offers one of the more thoughtful, comprehensive explanations of his vote that I've seen:
[W]hy am I voting the way I am?
Because McCain’s changed, I’ve changed, and the world’s changed.
How has McCain changed? In a nutshell, he’s gotten older. As we age, we get more set in our ways, less mentally flexible, more inclined to rely on narrative structures that we absorbed long ago. And the structures that dominate McCain’s brain are an almost perfect mismatch with our country’s needs at this time.
How have I changed? In a nutshell, I’ve gotten more conservative and less right-wing. (I’m using conservative in the sense of skeptical and cautious as well as in the sense of seeking permanence and valuing rootedness, and right-wing in the sense of believing in the importance of rewarding success more than in ameliorating failure.) More specifically, I’ve decided I was wrong about a bunch of things, and some of those things are Republican Party dogma, and much of that dogma is now central to the McCain campaign message.
How has the world changed? In a nutshell, the Bush Administration happened. We can’t pretend it didn’t. And whoever is the next President has to be responsive to that legacy. Perhaps, if McCain had been President in 2001, things would have gone better – or perhaps not. But we don’t get a do-over; we have to deal with the world as we have it and have made it, and not as we’d like it to have been.
You can read the whole thing here.