There's a lot to like in having Hillary as Secretary of State.
But there are a couple of problems. One, it seems like an awful snub of Bill Richardson, who (1) came out for Obama late in the primaries only to feel the wrath of Clinton and (2) represents a demographic that also came out heavy for Obama in the general election and contributed significantly to his victory, and which, one might think, is deserving of a prominent spot in the administration. A snub of Richardson would come across as a snub to many of them as well.
There's also that little matter of policy. One of Obama's signature themes, for which both Hillary and McLame attacked him, was his negotiating stance, his willingness to meet with one and all foreign leaders, promising, at least potentially, a break with the foreign policy norms of presidents past, Republican and Democrat. Real change if you will. I've no doubt that if she accepts the appointment, Hillary would act in Obama's interests here. But at a minimum, it could pose some awkward questions at a confirmation hearing--if there is one.