In addition to his insightful, courageous, and wise choice of Sarah Palin for VP, McCain earnestly admitted the errors that the GOP had made (repentance and contrition) on TV before a huge audience. People knew he was being honest and admitting the problem and promising to correct it are a large part of the dynamic here. People who were angry at the GOP for being the party of Foley (rightly so) and Bush (wrongly so but that's where the country is) are willing to give a McCain led GOP a fresh look and even chance.
Also don't discount the impact of the video story on Cindy McCain that preceded her speech. It was fantastic ! She will make a great First Lady.
A big AMEN to that! She has a great life story, and who can dislike a beautiful rich blonde that owns one of the biggest Budwieser distributorships in the west?