Sadly, the quality of the Simpsons has been trailing off in recent years (IMHO). It's still funny in a slapstick sort of way but not as clever. Some episodes do shine though.
I think a new "jump the shark" category should be when a shows cast goes on strike for more dough. Seinfeld, Freinds, etc all went down fast after their strikes. Sort of like the cast knows their days are numbered, and need to cash in quick.
I don't know - it had a slump there for a few years, but I've generally been enjoying the newer episodes much more. You should check out the current season and see what you think.