We live in an era of quite good stand-up comedy, which has a number of guild customs that strive to keep any individual from getting too large of a share of the market the way that Bob Hope had in the mid-Century. Jay Leno's ambitiousness grated against this spirit of the age, so much of the resentment against him and his mass market comedy came from intense comedy fans. The finest comedians tend to be messed-up depressives, but, still, I have to be in awe of the bulletproof supermen like Leno and Hope who can just go on and on and on.
Personally, I liked Leno as a comedian (I saw him live among nine stand-ups at the Improv in 1981; not surprisingly, he was the best, and his story about his father coming to visit him from Boston is one of the most memorable I've ever heard), and I didn't hold it against him that he pitched his show's comedy at the 98 IQ mass audience and let Letterman aim at the 103 IQ audience.
But, the interviews ... Unlike stand-up comedy, we don't live in an era of good interviewing, so the fact that Leno was a terrible interviewer didn't generate that much criticism. But he was much worse than his predecessor Johnny Carson. The science fiction classic Lucifer's Hammer by Larry Niven and Jerry Pournelle about the approach and catastrophic impact of a comet includes a brilliant chapter written from Johnny Carson's point of view as he interviews the two astronomers who discovered the approaching comet. Niven and Pournelle go inside Carson's head as he figures out on the fly on live national TV how to guide the scientists into making their esoteric topic fascinating to the masses in TV land. Granted, astronomy was Carson's hobby so this comparison is a little unfair, plus nobody is interested in anything besides show biz celebrities these days, but Carson was also much better than Leno at interviewing movie stars.