Here are my thoughts: the media piled on too much at the end and there was a voter backlash; independents may have assumed an Obama victory and went for McCain instead; the Democratic base responded to the Clinton appeal, especially women and urban voters; the youth vote didn't quite turn out as well as it should have; Clinton for the first time looked vulnerable and even human in the final days - and that helped; her final debate performance was also excellent.
She was knocked off her pedestal in Iowa and people prefer candidates not on pedestals. We will hear more about the Bradley effect, but I have no evidence it was actually there. I sure hope it wasn't. And South Carolina will give us more data.