I know I'm not the only one who doesn't find Leno funny. Most of his jokes are forced, and he often hangs onto them too long and even starts to explain them sometimes. When he interviews his guests, the conversation is hardly ever smooth and he isn't very quick on his feet to make funny comments. He's not horrible, but I don't believe that he is naturally funny. Conan and David Letterman are both naturally funny -- they have the wit and goofy physical comedy, but Conan has much more of both than Letterman. Needless to say, I'm really not sad to see Leno go... I've just been disappointed with his show too many times.
Conan O'Brien is way funnier than Leno. Plus Leno's jokes are dull, overdone and sometimes bordering on racial. Which has really turned me off to him. Letterman has gottin to the point where it looks like he just doesn't care. John Stewart is funny but at times he can be a bit dull. Leno should retire at the end of this year or next, he's pushin it. Alrhough his newspaper clippings/ads thing is way funny. The one thing that really keeps the show is the guests that appear on the show. In five years no one will be watchin him anymore.
Funny thing in all of this, I used to watch Letterman nightly (back in the early 1990s). I haven't really seen his show in a long time, so can't say what has or has not happened.
In addition to your tastes changing saz, it's also possible the show itself has changed (and perhaps definitely since I last saw it). Back then, some of these other comedians were the new kids on the block, and Letterman's main contender was Johny Carson (if I remember correctly), who has long since not been around...whereas Letterman's show had been around a bit longer.