For me, the reasons that he was also a great Surgeon General is summed up in these few lines in the ABC News item on Koop's passing.
Koop carried out a crusade to end smoking in the United States; his goal had been to do so by 2000. He said cigarettes were as addictive as heroin and cocaine. And he shocked his conservative supporters when he endorsed condoms and sex education to stop the spread of AIDS.
These were both very, very important things for the nation's top health official to do at the time. Especially when the President himself couldn't bear to say "AIDS" in public and many people still believed that smoking was just a 'habit', that low-tar and filtered cigarettes were safer and that the link to cancer had never been "scientifically proven" anyway.
RIP Dr. Koop