A pick that leans toward the cautious side of the equation.  He's a bit old, and a bit too connected to Washington, but he would please the conservative wing of the party without being the potential lighting rod that Huckabee or Brownback would.

Barbour wouldn't be a terrible pick, but I think he's generally seen as too close to lobbying circles and "official Washington". In this political environment, that might not been the best situation. Him paired up with McCain wouldn't match up too well; McCain needs someone younger and someone who is viewed as either a technocrat (to reassure voters) or a reformer (like McCain himself). Barbour doesn't meet either of those two criteria.

Barbour is politically experienced and has plenty of connections in Washington, which might be a positive. He would help the ticket in the South, but they don't really need it. Mississippi and much of the deep South are going Republican anyways.

This yayhoo from Yazoo City would balance McCain's ticket politically, to the delight of conservatives, also southern evangelicals, without raising the hatred antenna of the liberal democrat base too high.