Karl Rove is a fucking genius. This has probably occurred to some of you already, but this article makes such a good point that I feel compelled to repeat it here: while the Democrats were engaged in expensive and uncertain efforts to get out the vote, the Republicans managed to do the same thing at far less effort and cost by putting referendums on gay marriage on the ballot in eleven states, which guaranteed a massive turnout from evangelical Christians. Evangelical Christians showed up to vote against gay marriage, but they stayed to vote for Bush. And that's how he won Ohio. The referendums themselves will be challenged and probably defeated in the courts, but in terms of motivating the voters who were needed to re-elect the president, they succeeded brilliantly.

As Noah notes below, other factors, such as the "security moms" turnout, were certainly involved. Still, the gay marriage issue was a component of Rove's explicit turnout strategy that unambiguously had a tremendous impact on the election, and I think that most Democrats were blindsided by how well it worked. I sure as hell was.