I think I've pinpointed when The Simpsons began to go downhill. Season 9, episode 2(?) - "The Principal and the Pauper." The first truly horrid episode of the show, and the mark of its eventual downfall.
Season 11 is the first legitimately terrible season. 9 and 10 each have gems scattered throughout, but 11 is just ... awful.
Now, of course ... well, Duracell described it better than I could.