A comment on my previous post prompted a response from me, but it threatened to run on and on, and it occurred to me that it would be better served by a follow-up post.
In her comment, Lilli mentions simply not finding The Simpsons funny anymore, even when stumbling across a rerun. And sadly, I know that feeling: The very longevity of the show now threatens its legacy.
The prime years of the show, roughly seasons two to eight (some would extend it to season nine or even ten), remain in and of themselves as good as they ever were. But since The Simpsons continues to grind on and on, with the inevitable decline in quality, a feeling of fatigue is bound to set in. It's not even necessary to watch it to become tired of it. It's gone from Can't Miss to Is That Thing Still On, and it makes it difficult to watch even the prime years without some level of...well, "contempt" is probably too strong a word, but that about sums it up. An early Marge/Homer dustup reminds us how tired we've become of that recurring premise, just as an early, once-funny appearance by a now-recurring character (let's go with Disco Stu as an example) reminds us how subsequent years have pounded that character into the ground.
It doesn't help, either, that so many more recent programs have taken what was fresh and innovative and used it to their own purposes. Family Guy, most obviously, has taken some of the storytelling devices and gag set-ups from The Simpsons and used it to far cruder, less funny effect, but on a higher level, such shows as Arrested Development and Community bear a clear influence, and in these cases, the students may have outshone the master.
But that part is inevitable. Everybody takes from something else. Monty Python's Flying Circus once seemed daring and innovative, but now it seems almost sedate, TV as comfort food. But its quality, like that of The Simpsons, is absolute: No matter what, it's still good. It has nothing (except, perhaps, over-familiarity) to taint the viewing experience. But once we've seen Lisa Simpson lip-synching a Ke$ha song, it will never be possible to think of her the same way.