This & Hillary's appointment to secretary of state should put to rest any idea that he's going to be some uber-liberal president.
Gates is a good choice but, really, Hillary is almost as liberal as Obama.
And, just to play devil's advocate, let's assume that there are significant policy differences between Obama and Hillary, with Hillary to the right of the president-elect. By picking her either: (a) Obama expects her to come around to his way of thinking (which was, during the campaign, pretty far left on foreign policy); or (b) Obama expects to move to the right, thereby having broke yet-another campaign promise.
