Jonathan Rauch's National Journal cover story on Bush's legacy is well worth a read. His thesis:

Bush may go down in history as a transitional and comparatively minor figure. His presidency, though politically traumatic, may leave only a modest policy footprint. In that sense--though by no means substantively or stylistically -- Bush's historical profile may resemble Jimmy Carter's more than Truman's or Nixon's. Recall that in 1980 many people wondered if the country would ever recover from Carter. Five years later, he was all but forgotten.

But if Carter had had two terms ... ? Jon continues:

In other words, Bush may have accomplished something that seemed out of the question in January 2002, when he touched greatness, and in January 2007, when he touched bottom. Bush may have achieved mediocrity.

If that hypothesis sounds snide, it is not intended to. Had Bush left office at the beginning of last year, his tenure might indeed have gone down as calamitous. Winding up in the middling ranks, then, would be no mean accomplishment. Far from being happenstance, such a finish would reflect an unusual period of course correction that might be thought of as Bush's third term.