The Office
My completely unprovable but perhaps interesting theory about why The Office is successful: the creators are also the actors. That is, the actor who plays Toby (Paul Lieberstein) is actually the co-executive producer, Ryan (B.J. Novak) has produced many episodes, and Kelly (Mindy Kaling) is a chief writer. And don't forget Ricky Gervais -- the original "Michael Scott" (David Brent, in the BBC version) -- who is also an executive producer and writer. I would also bet that Steve Carell and John Krasinski influence the show's direction. This arrangement seems fairly unique to tv production. (I thought of this while reading a story in which Jeff Zucker suggests that The Office might turn into an hour-long show. Wacky.)