MSN has a very good piece outlining exactly what happened with Ms. Trump's plagiarism of Michelle Obama's convention speech from 2008. This fiasco has completely overwhelmed the dominant narrative surrounding the Republican Convention, which is a bit of a shame. While Ms. Trump committed a clear case of plagiarism, this is not a "fail the course" kind of plagiarism. I would, in fact, have overlooked it in one of my students-- had I even noticed it. Instead, I would have simply had a talk with that student.
According to the article, Melania decided to substantially revise the speech given her by the professional speechwriters, which I think is laudable. (And remember, even though she's been in the country a long time, English is not her primary language. I'm still impressed that my wife's English is so much better than many native speakers'.) And, furthermore, it's perfectly okay for someone to browse similar speeches from the past to find a model. That's what you're supposed to do. Melania just screwed up by sticking too close to the original in some places. This is embarrassing to the Trump campaign, sure, and you can't reduce this to a "teaching moment" with someone of this prominence, but I'm as likely to crucify Melania Trump over this as I am Doris Kearns Goodwin or Slavoj Zizek, who've committed similar slips.
About the most you can say about this incident is that it shows how slipshod the Trump campaign is -- and his tendency to skip established, good political practice.
I also wonder if this plagiarism story has taken over other potential stories, because all the other objectively more awful things being said on-stage at the RNC have begun to pall through familiarity. We've heard the casual racism and birther claims before; plagiarism is something new, even if people don't realize its relative misdemeanor status.