If Defense Secretary Robert Gates feels any twinge of wistfulness when he departs the Pentagon on Thursday, it probably won't last long. Even during the Bush years, Mr. Gates spoke often of the clock in his office by which he counted down the days until he could retire to his beloved Washington State. When President Obama asked him to stay on as defense secretary, Gates made no secret that he did so out of public duty, not an affinity for Washington, D.C. But Washington insiders certainly had an affinity for Gates. Here are three reasons America’s longest-serving secretary of Defense will be missed – and legacies that many hope will last after he's gone.