The "successful sitcom" I've always hated was Seinfeld.
There's not a single redeeming character in there. They're all hyper neurotic, lying, manipulative... every bad character trait I can think of. I wouldn't associate with anyone like them, and I had no interest in wasting my time watching people I couldn't care about.
That was kind of the point of the show, and what made it funny. In fact, the entire punch line of the final episode, which not a lot of people seemed to get (but I thought was brilliant) was exactly your point - these are terrible, horrible people you all have been laughing at or with for the last 7 some odd years. How do you feel about that? Its kind of the crux of Larry David's comedy, if you are familiar with it.