Hell, Leno giggles at his own jokes all of the time. Which I suppose you're right it goes okay in these types of shows as opposed to skit-shows like SNL. But SNL didn't endear me much to him so, yeah, I'm not willing to give him too much respect.
I fucking hated
Fallon on SNL, and his first eight months or so on Late Night were really
painful, because he was clearly having a very difficult time adjusting to the format, even worse than Kimmel (who was fucking terrible for a while). But for the last 18 months or so, he has been just killing it on Late Night. His material is solid, he's comfortable with it and the gimmicks he does have been great. Having the fucking Roots as his backing band helps, of course, but the complete turnaround Fallon has had is incredible.
Give him another shot. His show is a ton of fun, and a large part of it is because he's having fun on it, instead of having to play a character.
Conan was absolutely terrible in the beginning. His first monologue on Late Night was a jumble of bad timing, tripping over words, and almost visible trembling. It's burned into my memory like one of those bad dreams you just can't shake.
But he got better. It took a while, but he got better.