Michael Barone sums it up nicely, as he always does. One graph:
The framers of the Constitution created the vice presidency to solve the problem of succession. They expected that electors meeting in state capitals would vote for two candidates from different states, with the No. 2 vote-getter becoming vice president. It worked well twice. Then the unexpected emergence of political parties produced bizarre results.
My crystal ball is a bit dusty, but I would guess that Obama will pretty much have to select a woman, and McCain will need an actual conservative. That’s logical thinking to my mind, therefore I will probably be wrong on both in this election year.