I would use the comparison of Bill Clinton.
Nixon didn't do that much apart from getting out of Vietnam. This was overshadowed by Watergate as the all-time Presidential scandal.
Clinton didn't do much except mess about a bit with Bosnia and Northern Ireland. This was overshadowed by be nearly getting impeached - although with hindsight and all the stuff that came out later it all looks a lot worse. So - the common factor is that the two presidents didn't have anything larger than their scandal.
Compare this to say Obama - nothing will ever be bigger than his ethnicity. Compare this to Reagan who saw the end of the Cold War. Compare this to HW Bush who had the Gulf War, W Bush who had 9/11 and Iraq. JFK getting assassinated. Truman dropping the bomb. For all these Presidents, they have maybe one defining moment. For Nixon it was Watergate.
Think about Presidents like Ford and Jimmy Carter. No one remembers them because nothing really huge happened.