He smoked like a chimney. He said he was up to 4 packs a day and couldn't remember how long he went on at that level.
I watched him even when I was younger. He was funny, knew how to rib someone in a classy way - knew how far to take a joke, and had a sense of comedic timing at a level few possess.
He also was someone who seemed to value friendships, and went out of his way to let his friends know they were appreciated. I believe no host today to possess the same personal qualities Johnny did, which is also why the need to go to the lengths they do to attract an audience. You watched The Tonight Show because it was like hanging out with a friend and just enjoying the evening before heading off to bed.
The closest comparision in his field I can think of is Jack Benny.