From Tod Lindberg in the Weekly Standard:
Nobody ever said Barack Obama was unqualified for the No. 2 spot on the ticket.
Had Hillary Clinton won the nomination and selected Obama as her running mate--which, being a savvy politician, she would certainly have done, in order to fire up his 18 million primary supporters--Obama would have been perfectly positioned. Either he would be preparing himself as vice president for his run for the Oval Office eight years hence. Or he would be experienced and tested in a national campaign that he would never be held responsible for losing, with a fundraising base beyond the imagination of Croesus.