Michelle Obama's speech presents a specific worldview, which Barack Obama would fulfill by becoming the next president of the U.S. The first principal of rhetoric—advisory—is precisely this: advice to the audience to adopt a particular worldview. In this case, Obama represents and promises the fruition.
The second rhetorical principle—addressed—has to do with who the audience is and who the speaker perceives the audience to be. Michelle Obama uses words and phrases like "as a mom," "as a daughter" etc. The anecdotes and examples she employs are embodied in the heroes of her narratives: "blue collar workers," families hard pressed for money but instilled with the kind of desire that drives them to accomplish seemingly impossible jobs so as to provide for their children and make ends meet. She references her own family and her relationship with Obama, appealing to the personal and relatable situations of those in her audience. She spoke to Americans, voters, those concerned with their livelihood and their children's futures. Her choice of words and analogies make this obvious.
Her rhetoric arises out of a lack the audience experiences--be it socially, financially, etc. Her speech relies upon the assumption that there is a palpable need for change and hope felt among her audience and all of America. People are working hard and not always finding help from the government or even acknowledgement of their dire situation. There is a need to recognize this national "other" that Obama promises to see.
Finally, her performance was especially influential. She delivered her speech in a manner similar to that employed by Obama: gestures, voice, tone, expression. She referenced the anniversaries of two momentus events: women gaining the right to vote and MLK's speech. The images tied up with the events will call to mind already accomplished milestones of a similar sort—things to which to aspire.