Both men look very weary, and who can blame them; and perhaps because of an inner "enough, already!" mood Obama is being less controlled about little smirks and shaking his head 'no-no when McCain says something he doesn't like. McCain has not controlled his disdain for Obama in previous debates, and he's not even trying now.
But the ten minute or twelve minutes that began with Obama looking at McCain and talking about crowds at Palin rallies saying "Kill him" were riveting TV and seemed to reveal purified versions of the persona each candidate has been presenting through the previous sessions. This debate may matter less in the long-term outcome than the others, since that's typically true of final debates. But because the contenders are engaging each other more directly -- being at the same table, being physically so close to each other, having more trouble containing their emotions, being aware that the whole thing is almost over -- in human terms this is actually the most interesting.