The critical exchange came early, and McCain won it because he has the truth on his side.  Obama did not answer McCain's stinging assault on Obama's and his party's complicity in the financial meltdown brought about by Fannie and Freddie.  Obama's statement that "I never supported Fannie Mae" is an invitation to MSM to examine his real relationship with this financial crisis and its prime movers. This exchange need to be replayed again and again, as does McCain's comparison of Obama's tax and protectionism policies with those of Hoover's. 

The rest of the debate is a non-event which favors Obama since he is ahead, except perhaps for the question on Iran, which has to always help McCain with American voters who support Israel and the close where McCain reminds voters again of his seriousness and experience.  Indeed the final 30 minutes turned back to foreign affairs in a repeat of the questions of the first debate.  Obama fluently presents defeatism as an acceptable strategy, and McCain's strong suit is his experience, but he did not attack Obama directly and missed opportunities to do so. 

Bottom line: McCain was after Fannie and Freddie, and Democrats including Obama did not.

UPDATE:  During the debate, North Korea fires off two missiles, contributing more to the debate than the questions from the aduience or Brokaw, by underscoring the stakes involved here.