Here's an interesting discourse on why America prefers Jay Leno to David Letterman. Personally, I think it may be a little simpler than some make it out to be. I think a lot of folks feel Dave is doing the same show over and over, and that he often exudes the traits of a man who doesn't want to be there…whereas it's just more pleasant, especially around bedtime, to be at Jay's party. Those criticisms are now less true than they were for a while there — I think Letterman is a lot better lately than he's been in years and Jay's fallen into formula — but the shifts have been subtle. I still watch 'em both but find myself doing frequent Fast Forwards on the TiVo remote, usually when Jay turns things over to a guest correspondent or Dave cuts to Rupert's Deli.
One of the skills Mr. Carson employed to keep his show fresh for three decades was a good sense of when a bit had gotten tired, and he did not base these decisions primarily on the response of the studio audience. Johnny knew that folks who come to see you tape are (a) more devout fans of the show than average and (b) apt to laugh out of recognition at a segment that would have triggered déjà vu if seen at home. He often retired bits or removed stars from the guest list even though the fans in the seats seemed to like them. One of his writers once told me that Johnny was suspicious of any segment that felt too easy. One of the downsides of Jay and Dave both working so many nights without vacation or guest hosts may be that they can't afford to drop anything that is easy.