The noteworthy aspect of Obama’s ensemble is that in recent history, first ladies have rarely dressed so informally in public, particularly as they are emerging from Air Force One while a phalanx of photographers stands ready to record the moment. This exclusive group of women might have dressed in a relaxed manner — khakis or jeans, for instance — but it was always in a way that suggested that they were keenly aware of the ever-present cameras. None of them revealed as much leg as the current first lady, either — a fact that has been duly noted on the Internet by a nation that gets more squeamish about an artfully photographed nude than it does over a naked body lying in a pool of fake blood on an episode of “Law & Order.”