I think Dave can sometimes make his guests uncomfortable . . . and he can be perceived as laughing AT his guests, instead of with them on occasion.
And I think that whole Letterman-Oprah "feud" started at Dave's ill-fated attempt at hosting the Oscar telecast, didn't it? The infamous "Uma/Oprah . . Uma/Oprah . . . Uma/Oprah" dialogue.
Anyway, both looked like they were genuinely having a good time.
Wonder when Dave's going to be on Oprah's show now?