As far as I can tell, Thomas's tenure on the bench is just what was expected of him: a reliably conservative vote--in the political sense of the word "conservative." Whether or not his life story conditions his votes (how could it not?), his politics (again, in the narrow sense) do. He may have a fully articulated, deeply felt and intelligent way of linking the political outcomes he achieves with some deeper set of principles, including a judicial philosophy, but when politics and principle conflict, the politics have much more to do with the outcome than the principles.

Example: Bush v. Gore.

Thomas is not, by any means unique in this respect, and I'm sure there are many liberal judges, distinguished and not, of whom we can say the same. (And it's not, in and of itself, a bad thing. It's how people make decisions. To suggest that we can or should make our decisions based on principle all the time is empirically thick-headed, and in this instance, manifestly partisan.)

However, the fact of the matter is that Sotomayor, on paper, is far more qualified for a nomination to the Supreme Court than Thomas was when he was nominated. Judicial experience? She has more. Academic success? She did far better at every step. She's even been more successful in the private sector. The only area where Thomas exceeds her is in executive branch experience: he was a Republican appointee in Republican administrations.