Aside from their tenure as elected officials, keep in mind that Obama announced his candidacy in January of 2007. he has spent more than 18 months under a microscope. And he convinced millions of voters nationwide that he was the best candidate. In short, he has the experience of earning the nomination.
Palin, in contrast has lived her whole life far from the klieg lights. And she now has the experience of having the brass ring handed to her.
2. The key here is indeed as others have noted, the appalling lack of judgement McCain has shown. Part of the reason VP candidates tend not to come as such a left-field surprise is that publicity tends to surface defects that can and should disqualify a candidate. Those defects may be old scandals, or they may be about a lack of grace in handling the pressure. But that process can save (and I'm sure has saved) nominees considerable embarassment.
None of that has happened here. When the dirt is exposed, and it will be, McCain's campaign will detonate. I think the reason Obama's campaign quickly backed off their criticism is that they know that Palin is a house of cards, and they won't need to do more than breathe to knock it over.
3. The Rebublicans will insist that Palin is Obama's mirror image. But I think Palin's democratic equivalent would be Gavin Newsom (or would if Palin were better known). San Francisco is actually slightly larger in population than the state of Alaska. Newsom is a hero to the left, just as Palin will be to the right. Neither has any national or foreign policy cred. And just as Newsom is tainted by minor scandal that mixes the official with the personal, so is Palin.
Think about how the talking heads would have reacted if Obama had picked Newsom.