There's something going on akin to the 1960 "Missile Gap" issue - Kennedy said that the Soviets had more missiles than the US, and Nixon, being the Vice-President, knew this was untrue but knew that he couldn't say so because the information on missile numbers was classified.
Similarly, Obama can't acknowledge the very successful cyberworm attack on Iranian nuclear facilities (which the administration denies but there are books, articles etc about it), nor can he mention whatever number of covert operations are in progress. Consider, for example, if this election had occured in April, 2011, when Obama knew Bin Laden's location and was planning to execute an operation against him - Romney could have hammered him for not having caught Bin Laden, and Obama would have had to keep quiet.
Beyond the Iranian worm attack, I can't really begin to speculate what kind of stuff Obama's hands are tied on, but it's an interesting angle to consider.
That said, Obama looks strong on foreign policy. It's unlikely that any of the real "third rail" issues will come up, just like they didn't in debates one and two - surveillance, drone attacks and shadow wars. If they do, expect them to be dealt with superficially.
By the way, anybody who has not read Bob Woodward's book about the Afghan strategy review, Obama's Wars, is doing themselves a disservice.