The Simpsons has been running so long for a myriad of good reasons. The storytelling is great and multileveled, which makes re-watching episodes a joy instead of a nuisance. The characters have developed quite a lot over the years, while still maintaining the core of their personalities, much as a real human being would. The only difference is that they don't actually age. If they did age, Bart or Lisa (the timing of their births is a little weird and hard to pin down exactly) would be my age. I think one of the success secrets of the show is the acknowledgements of the fans that they slip in every now and then--referencing past episodes in later ones, openly questioning things as a fan would ("Which Springfield?!" "Wait, if Marge is 34 and Bart is 10, then she didn't have him right after high school!" "'I thought you were dead.' 'No, I've just been gone for a while.'"). It's a geeky show with a main stream appeal. The Simpsons is the first family show we shared with our daughter and she's just as hooked as we are.
What appeals to me the most, however, is the fact that I can use the show to illustrate concepts in my classroom. There is at least one episode for every major topic in psychology. Stress and coping have several episodes ("Simpsoncalifragilisticexpiala[annoyed grunt]cious", "Hurricane Neddy", "Make Room for Lisa" are just a few I use in my classroom). There are many that concern therapy and mental disorders (e.g. "Stark Raving Dad", "There's No Disgrace Like Home", "Fear of Flying"). I have found episodes that are appropriate when discussing intelligence, critical thinking, work and team development, vocation selection, parenting, morality, compliance, conformity, obedience, cults, mob rule, hypnosis, drugs and hallucinations, sexuality, gender issues....The list goes on and on. I'm pretty sure that I could design an entire psychology course centering around The Simpsons and still get the point across to my students. In fact, I hope to do just that some day, when I'm not too busy working on my pet research project or teaching classes/grading papers. Sadly [or maybe, since this is an optimistic blog, luckily], only the first 14 seasons (and season 20) are out on DVD at the moment, and some of the more recent episodes fit in to some topics better than older ones. Yet, I still try to incorporate an episode in my class whenever I can. It breaks up the monotony of lecturing. Infusing humor into a class also helps my students see a lighter side to these sometimes heavier subjects.
Perhaps the show works so well as a teaching tool because it doesn't try too hard. I'm sure the writers did not design the episodes to work hand-in-hand with my lectures. They just naturally fell into place that way. I mention many other shows in my class and many characters, books, movies, etc., as illustrations of the topics, but nothing is used more often than The Simpsons. It's just endured for so long for so many good reasons. I would not mind it sticking around for another 20 years, as long as the quality of the storytelling stays at the same level it has since the beginning.