Since I'm trying not to do my homework (and succeeding famously), I figured I'd go about applying the Small-Town Mayor Test to the 32 candidates listed here. At least a "maybe" is necessary to continue.
Could they be a mid-western small town mayor?
Vilsack? Yes. In fact, he was.
Zinni? Yes. His hometown is tiny.
Ford, Jr.? Maybe
OK, so that knocks out Kerry, Dodd, Sebelius, Bloomberg, Napolitano, and Bradley, a few likely short listers, and a couple of names you'll never see anywhere near the short list. Now, I knock off other names that are simply implausible for one reason or another: Clinton (obvio), Edwards (said no), Ford, Jr. (black and conservative), Hagel (Republican), Nunn (dude'll be 70 by election day), Powell (black and conservative), Gore (obvio).
OK, that leaves us with Sens. Bayh, Biden, Casey, Cleland, Daschle, Lincoln, McCaskill and Webb; Govs. Bredesen, Kaine, Rendell, Richardson, Schweitzer, Sebelius, Strickland, Vilsack; Reps. Gephardt and Roemer; and Gens. Clark and Zinni.
For the next elimination round, they've got to be convincing as potential commanders-in-chief.
All 17 who got at least maybes are viable running mates. But to get to the top tier, one needs to have something extra. A state or a region where they help enough to add EVs. A characteristic like foreign policy experience that rounds out the ticket. Something big.
Who's got that something extra? Biden (foreign policy, national security), Cleland (fp-ns), Webb (fp-ns, post-partisanship, maybe VA), Kaine (outsider, maybe VA), Rendell (PA), Richardson (fp-ns, NM), Schweitzer (outsider, maybe helps in NV and CO), Sebelius (woman, outsider), Strickland (OH), Roemer (fp-ns), Clark (fp-ns), Zinni (fp-ns). There's my top 12.
Of the governors who only get in by virtue of being outsiders, I prefer Kaine over Schweitzer, who I'm not sure has any real effect in the West. Of the generals, I prefer Clark over Zinni, for having actually been a candidate before and spending quite a bit of time stumping for Democrats all over the country. Of the former members of Congress who get in for national security, I prefer Roemer over Cleland for his relative youth and progressivism and his 9/11 Commission membership. I'm also knocking off Ed Rendell because his machine politics would conflict with Obama's message and the dude's already gonna do everything he can to win us PA, because he gets off on bragging about it.
So, my 8-member short list would be: Sens. Joe Biden and Jim Webb; Govs. Tim Kaine, Bill Richardson, Kathleen Sebelius, and Ted Strickland; Rep. Tim Roemer, and Gen. Wesley Clark.