I think that the gay marriage question could disqualify a lot of the potential nominees, from the conservative viewpoint.
Currently, the Constitution does not ban gay marriage. So they would all say that it's a state issue, which would not sit well with conservatives.
Of course, I suppose a liberal might think that the Constitution bans discrimination against gay couples wanting to get married, or that the Congress cannot legislate on it.
Hopefully none of the nominees Bush might put forward would say that, though.
The Constitution didn't ban "sodomy" either but Justices Scalia, Thomas, and Rehnquist were the only ones who DISSENTED on the SCOTUS sodomy ruling:
http://en.wikipedia.org/wiki/Lawrence_v._Texas (legalizing sodomy)
Scalia, Thomas, and Rehnquist just happen to also be the only judges who joined O'Connor in dissenting on the recent eminent domain case:
http://en.wikipedia.org/wiki/Kelo_v._New_London
(eminent domain)
It seems the main conservative issue that Justice Kennedy is good for may be "affirmative action" issues? Justices Stevens, Souter and Ginsburg appear to be pretty far out in left field by comparison:
http://en.wikipedia.org/wiki/Grutter_v._Bollinger (upholding Michigan University's right to use race as a criteria for admissions)
Rehnquist, Scalia, Kennedy, and Thomas dissented
http://en.wikipedia.org/wiki/Gratz_v._Bollinger (striking down Michigan University's point-based system of affirmative action)
Stevens, Souter, and Ginsburg dissented
Huh?