It seems like each person that's been mentioned has something about them that could be a problem.
For example -- Romney's flip flopped a bit too much. Jindal and Palin are too young and inexperienced. Nunn is too old. Crist is too single. Biden and Dodd are DC insiders. Pawlenty and Easley are not well known. Romney and Clinton don't get along with their parties' presumptive nominees. Clinton and Gingrich are, well, Clinton and Gingrich.
There's no one perfect problem-free possibility for VP. What McCain and Obama have to ask themselves if the pluses outweigh the minuses.
Neither is Nunn, Crist, Biden, or Dodd. You have to be aware that most Americans don't know any politicans outside their homestate except sitting & former Presidents & VPs. Being picked causes your name ID to instantly shoot up to 95% anyway.
I will have to say though that with the exception of Pawlenty, Sanford, & maybe Thune, everyone that McCain is considering would be really awful to pick, many for reasons that aren't their own fault.
Instead of "not well known" I should've said "virtually unknown". That would apply to Pawlenty, Easley, Sanford, Crist, and Thune. Nunn might take a bit of memory jogging, but I would imagine that Biden or Dodd might be a little more familiar having recently run for the Democratic nomination.
Even so, I doubt we'll ever see another complete unknown a la Dan Quayle, thanks to 24/7 news coverage and the internet.