I think the Simpsons jumped the shark when every episode started to revolved around Homer and his increasingly "crazy" stunts, basically sidelining all the other characters. The Simpsons used to be a show about a family and now....most of the episodes in the later series are about Homer becoming an artist/missionary/trucker/Burn`s Buttmonkey etc. It used to be that Springfield had a flimsy grounding in reality, which allowed situations to evolve naturally. Now its about Homer doing something mad that actually makes no sense even in the context of the episode, whilst interacting with gratutious celebrity appearances shoehorned into the episode by the writers.