Just briefly, after a week: His musical numbers and thank-you-card schtick were (on "The Late Show") / are (on "Tonight") cute and dumbly audience-friendly. A la Carson. And I HATED Carson. I'm completely of the "Letterman Generation": Weird and cranky. To me, Fallon's schmoozing with Justin Timberlake is the same as Carson's schmoozing with Buddy Rich. That "insider" wink-wink/aren't-we-hip thing was cheesy in Carson's day and it's just as cheesy now that it's been resurrected (not just by Fallon, but by the Entertainment Media as a whole).
Letterman was a talk-show groundbreaker when he debuted in 1982 because he was clearly outside of the "industry club"-- he didn't suck up, his skits were freakish. When I, as an 18-year-old, first saw him on TV, I was astounded and relieved at the anarchy and non-bullshit.
And now, with Fallon, we seem to have come full circle back to the phony-nice-guy/pseudo-hip Carson model, where every guest, however inane, is bowed to simply because of the mere fact of their celebrity. Letterman used to slyly point out such guests' inanity. Fallon, on the other hand, tries to completely disguise it -- the TV equivalent of veneers or boob jobs.
No one wants "mean-and-nasty" as they're drifting off to sleep. But I think that most of us also have an internal "The Emperor Has No Clothes" investigative impulse when it comes to the rich and famous, who almost certainly had to develop utterly false personalities in order to appeal to the powers-that-be who hired them...and to appeal to a mass audience.
Letterman's legacy was/is the tweaking of the façade. Fallon, on the other hand, sinks us right back into the "aren't celebrities great" morass so prevalent on TV prior to Letterman's psychological/intellectual breakthrough in 1982.