One of the lessons from McNamara's tenure during the Vietnam war that seems useful today is how much stock Washington puts in concepts such as "credibility" and "resolve" - even when pursuing these ephemeral concepts leads to the detriment of tangible power. During the Vietnam war, feeding more and more U.S. combat power into what was increasingly viewed as a losing effort was deemed necessary to shore up America's credibility as an ally in the Cold War. Washington was willing to sacrifice real power to prop up the perception of power.
This from Time's obit confirms the dynamic:
McNamara admitted in his book that the U.S. government had never answered key questions that drove its war policy such as whether the fall of Vietnam would lead to a communist Southeast Asia and if such an occurrence would really have posed a grave threat to the West. "It seems beyond understanding, incredible, that we did not force ourselves to confront such issues head-on," he wrote.
It's not clear yet whether the Obama administration has confronted these issues with respect to Afghanistan and Iraq.