Honestly, the US was probably "not ready" for a black president in 2008, but Obama managed to transcend race, which was a pretty herculean effort. But he was simply a stonger candidate than McCain. Likewise, I do think that Hillary being a woman is a net negative in itself, but the quality of the candidate herself is still by far the most important factor. Anyway, I see zero reason why a double female ticket should fare worse than a female/male ticket. No voter opposed to the idea of a female president would vote for her anyway, just because her VP is a male.
I think in 08/12 many white people were looking for any excuse to NOT vote for Obama. That's why in an election that should've been a Goldwater/Mondale-esque stomping (incumbent Republican at 25% approval, economic collapse, disastrous and unpopular war, horrible VP pick) that it was only a modest landslide. I still think Edwards (minus the affair) or some other charismatic (preferably Southern) populist white guy could've won that election with ~450 EVs and a double digit popular vote margin.
In 2012, whites went against him en masse by the biggest margin since Reagan in 1984, but demographic changes still gave him a comfortable victory. The big story and decisive factor of this election will be whether or not the Republicans hit their ceiling (or close to their ceiling) among whites in 2012.
