In my opinion, Carla peaks in "Truce or Consequences".  She is actually likable for the first and last time in the series.  As little as that is, it allows me to tolerate her.  Otherwise, her treatment of Diane is enough to make me despise her.  I love when Diane gets to shred her, however rare it is.

Terrific analysis.  Anytime someone has something to say about Cheers, I'm all over it, but this was thoughtful and insightful.  I do have to argue the premise that the Sam and Diane relationship failed because of their fundamental differences or because everything had been "done".  I think they could have gone on…