I don't want to say that the Christmas special is that much more offensive that the TV show—it isn't, by much, really. There's one scene that centers around Charlie asking Santa a question over and over, a question that includes the F-word, and the conversation devolves into something a little more gruesome. (It's hilarious; they previewed this scene at The Nightman Cometh live stage show in Seattle in September and it went over like gangbusters.) And there's a scene with Danny DeVito and a sofa, which would probably require a blurry spot to be shown on TV. But otherwise, it doesn't really go that much further—how could it? The show already takes it as far as you can go with a live-action sitcom.
A Very Sunny Christmas has a plot, about the magic of Christmas or something, but whatever. It's kind of a perfect marriage between subject matter and the tone—Christmas cheer vs. the characters' borderline psychosis. Perhaps thankfully, Jesus is left entirely out of the mix, except for a cameo by David Huddleston (who was the titular character in Santa Claus: The Movie, but who you probably know better as the other Lebowski in The Big Lebowski) who plays Danny DeVito's born-again former business partner. And Pablo Schrieber (Nick Sobotka from The Wire) has a good cameo as a classmate of Charlie and Mac's.
The double-length episode (roughly 45 minutes on the DVD) stretches the show's freneticism for a little too long, and childhood flashbacks of the gang's Christmas pasts fall flat, but otherwise most of the jokes work. Best bits: Charlie plays Simon (the game). Frank and the sofa (I won't say anything else about this). Charlie confronts Santa. A Very Sunny Christmas is funny enough to fit in with the canon of the show, and it's good enough to warrant a repeat viewing at Christmastime next year. It probably won't become a tradition like Charlie Brown or the Grinch, but hey, you never know.
(Bonus: the "Christmas sing-along" bonus feature is one of the weirdest, most disturbing things I have ever seen.)