Romney has come out of a very difficult Republican Primary, where he debated powershouses like Newt almost two dozen times. He started as a weak, timid debater; now, he's much more polished, much tougher, and much more on point. I don't think Obama has faced this kind of debate challenge before.
Plus, what successes, factually, does Obama have to argue?
None.
***On edit, there is the Osama Bin Laden decision, but that's about it.***