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It wasn't a policy speech. It wasn't a speech about the focus of an Obama presidency. I know of absolutely no one who has changed their minds on who to vote for based on a speech made by their spouse.
Speeches like this are pretty pointless to me and tend only to reenforce the views already held about a candidate. Those who already support Obama are going to listen to his wife's speech and have their positive feelings validated. Those who don't support him, or have questions about his qualifications for the office he is aspiring to probably aren't going to be swayed by his wife saying what a wonderful man he is. It's political theater - nothing more.
Look at what happened with Teresa Heinz-Kerry - those who supported Kerry liked her outspokeness while Republicans painted her as a Hillary-type bitch. A political wife is expected to speak nicely in public, not give too much information about her own opinions and show unending support for their husbands. In that, Michelle Obama accomplished her objective. Still, in the end it really means very little.