This was certainly a disappointing result -- from both a policy and a forecasting perspective -- especially considering that the vote wasn't actually all that close. It's one that's worth a little self-reflection.
There are essentially three conclusions that we might come to in evaluating the model's performance:
1) It was a basically good model that got a little "unlucky";
2) It was a poorly specified model that missed important factors in play in Maine;
3) It was a decent enough model, but missed some sort of national backlash against gay marriage.
Silver says that while the third conclusion is tempting, he doesn't think polling data support it. Is there, in his phrasing, a "national backlash against same-sex marriage?"