Many professional comics consider Jay Leno one of the best stand-up comedians of his generation. In his prime, Leno was a brilliant craftsman with sharp material, impeccable delivery, fantastic timing and an affable everyman persona. Even future enemy David Letterman had enormous respect for him as a stand-up comic in the beginning.

So why has seemingly the sum of the comedy world turned on Leno with shocking viciousness? Why has he raced past the reviled likes of Dane Cook and Carlos Mencia on the list of popular stand-ups...