What sets Carson apart from the rest was his ability to conduct both a serious or a comedic interview. And he was good at both.
Since Carson, the networks have chosen only professional clowns to do late-night. They don’t have anyone who can mix it up a bit. Leno can’t. Letterman can’t. Don Imus does both types of interviews, but sucks at both.
Say what you will about the guy, but Howard Stern is one of the best interviewers out there.
In addition, unlike these others, Carson was never ideological, petty or just plain mean.