I'm sorry. Someone has to stand up and say, "y'know, Letterman hasn't been funny in 20 years" and no one else has done it, so it might as well be me.
I was a fan back in the day, I really was, but the last 20 years he's been coasting. He comes out, makes a reference to the pre-show warmup that the studio audience thinks is hilarious but none of the two million viewers at home do. Then he makes the "it was so cold in New York today, in Central Park I saw a squirrel warming his nuts" joke that he tells every night. Then he'll repeat the punchline again and again, substituting Tourette's for comedy.
Then he'll say, "Introducing a new segment on our show, 'Hillary Clinton drops a pencil," and they'll cut to a title card that says "Hillary Clinton drops a pencil," with generic music, then a two-second clip of Hillary Clinton accidentally dropping a pencil, then back to the title card and music, then cut back to Dave and the audience is laughing hysterically at .. what …?
Then he sits at his desk and clears his throat for ten minutes.
Serious, he'd sit at his desk and clear his throat for ten minutes, or just make more Tourette's sounds. Then he'd ask "does anyone have a lozenge?" And he'd repeat the word lozenge for ten minutes.
In all the tributes I've read they all say the same thing: "remember when Dave did that real funny thing 30 years ago?"
It's been unwatchable for about 20 years now.