The show has one big, BIG problem that keeps it from remaining as fresh as it could be: It's essentially corporate welfare for the Harvard Lampoon Mafia. If you knew the right people and hung out with the right crowd at Hah-vahd, you WILL get a job on the "Simpsons" writing staff. This guarantees a stagnation of people who all think and act alike, hang in the same social circles, etc. It's why the show has been trending downhill for years; these people have said all they have to say, and thus the plots are becoming increasingly stretched and contrived. What they ought to do is keep one or two veterans on as head writers, and then fire everyone else and replace them with people that have no Ivy League connections whatsoever. Fresh outlook = fresh writing.
Addendum:
1) Even with this massive problem, they've managed to keep the show at least tolerable for over ten years now, which is an achievement. I just think it would still be amazing, instead of merely so-so, if they'd do what I suggested above.
2) For all its faults in this area, the absolute "worse. episode. ever." is Citizen Kane compared to the best episode of "Futurama," which is corporate welfare for the Harvard Lampoon Mafia Second String. (And if you've ever actually read a copy of the Harvard Lampoon anytime since, oh, the early 70s, you'll realize that 95% of those "comedy geniuses" are not any better at what they do than you and your buddies were when you got together during study hall in junior high school to tell jokes and make fun of your teachers.) To get an idea of how bad "Futurama" is, note that they gave a writing job to one of Al Gore's daughters.