I'll agree insomuch that a lot of it has to do with the candidate. But, Carter wouldn't have won if Ford wouldn't have pardoned Nixon, at least I believe that's the conventional wisdom. Even with that, Carter won in a squeaker.
Clinton only garnered 43% of the vote, I would argue that he wouldn't have won if it wasn't for Ross Perot.
And as you point out, both Clinton and Carter came out of no where, much like Obama. They didn't have any political baggage on the national stage in the same way that Palin does. Carter, Clinton and Obama were very much blank slates upon which many moderate voters projected what they wanted to see. Palin is anything but a blank slate, or canvass. If she can't repair her negatives, she can't win a national election.
I still contend the best way to mend her image is to spend a term in the Senate, building her credibility with middle of the road voters.
Popular Governors make great candidates, out of work guys like Romney should go buy himself a senate seat somewhere.