Moral Equivalence: what is it and how can I avoid it?

Moral equivalence — how everyone hates it! I'd never really come across the term before but have soon learned that the quickest way to defeat an argument in American politics is to accuse your opponent of moral equivalence, then sit back and watch them unwind. So what is moral equivalence and how can I avoid it? Apparently Obama's speech in Cairo was full of it:

Another example of moral equivalency: “In the middle of the Cold War, the United States played a role in the overthrow of a democratically-elected Iranian government. Since the Islamic Revolution, Iran has played a role in acts of hostage-taking and violence against U.S. troops and civilians.” That is accepting the (false) narrative of the Iranian Revolution, which holds that America’s role in overthrowing Mossadeq more than half a century ago — a development that would not have been possible had the leftist prime minister not lost support in the Iranian street — is just as bad as the campaign of mass murder and kidnapping that Iran continues to support at this very moment.

Ah. I see. It means taking umbrage at the most featherlight critique of American foreign policy. Obama did not say America's overthrow of a democratic regime was "as bad as" Iran's current day activities. He simply pointed out that America' record was not unblemished. That, apparently, is moral equivalence. Someone says something minorly critical of America's record, you jump in, infer value judgments that are not there (he's saying we're as bad as the nazis!) and then accuse your interlocutor of making them. Hmm. I will have to try it some time.