If the Simpsons "Jumped the Shark", I think they managed the rare of feat of jumping back.
Season 8: "Homer's Enemy" was a decent episode. I have no great objections to it, but it seems to me that it signalled that the writers were moving Homer over the top. It had happened sporadically before, but after this episode the writers continued on this trend of adding spontaneous and temporary rewrites to character backgrounds with consistent extreme behavior from the central character--usually Homer.
While some consider seaons 9 one of the better seasons, it nonetheless contains episodes which are jarring and stretch the standard characterizations to bounds not previously seen. For example:
The Principal and the Pauper totally rewrote a major recurring character and then subsequently abandoned it. One of the strengths of any long running show is the ability to slowly unfold hidden secrets of side characters. This particular secret came almost out of nowhere and disappeared just as quickly.
The Cartridge Family and Bart's Star are both acceptable episodes but they continue the trend of dropping subtleties for blatant and extreme characterizations.
All Singing, All Dancing is a clip show, but it starts an annual trend of clip shows and the like.
The rest of the season is extremely average with the exception of "Joy of Sect" and a couple of other good episodes. A couple of groaners for me would include "Simpson Tide" and "Trash of the Titans"--both centered on extreme and single-minded behavior from Homer. "Natural Born Kissers" most certainly had its moments, but it took its ideas way over the top in the final scenes.
Seasons 10 likewise contained some decent episodes but the tendency for the writers to go over the top was consistent. Unlike previous seasons, this one contained no outstanding episodes to redeem it.
The start of season 11 almost made me decide that maybe the Simpsons were no longer worth watching. "Missionary:Impossible" was the first episode that gave me hope that the Simpon's could continue their previously high quality episodes. Season 12, indeed, was one of the best seasons in their history.
Season 13, unfortunately, did not continue the trend. There were not too many terrible episodes, but there were few were close to being the gems that ususally show up. For me, the best ep of the season was "Papa has a Brand New Badge".
My serious thoughts on the Simpsons. But after all, it is only a TV show--even if it's something I've looked forward to for the last 8 years. . .