What made "Kotter" great was that it was just plain funny. Unlike some of its contemporaries, it didn't try to shock or send a social message. The four unique personalities of the Sweathogs, along with Kotter, worked together so well. The episode where they ran a telethon from the classroom was classic. The final season was a disappointment because the show's writers wanted to go in a different direction--attacking more serious issues. Plus, the departure of John Travolta after he hit it big in the movies hurt "Kotter" as well. It's probably all for the better that it only went four years because I think when a sitcom goes for too long, it tends to deviate from its originial concept (i.e. "Happy Days" and "Laverne and Shirley").