The first time I ever noticed a real change was in the first episode of season nine (1997 - 1998.)
Its been years since I've seen that episode now, but even now I can remember that the episode just felt different. Lots of stupid gags like the crab juice, potty dancing and groaning, running up and down the towers looking for a toilet, the guttural moaning, the first appearance of the idiotic "Duff Man," and the relatively incoherent reason why they had to go to NYC in the first place just so they could get their car back.
There had been some hit and miss episodes up to that point, but that whole episode simply
felt different and the rest of the season followed that trend. My attention dropped off over the years after that as I quit making a deliberate effort to tune in to my favorite family each week, but I finally shut them off for good sometime early on in season 12 (2000 - 2001.) I think it was at or around the episode where we find out that Homer is as stupid as he is because of a crayon lodged in his brain.
I realize that the show goes on, evolving over the years. They've done what they've done, they're covering new ground. Changing a bit because of other shows and different writing styles, new stories and new angles. But it still breaks my heart to think back to how deep the town of Springfield used to be, and how rich and detailed each character was.
The most obvious examples being the family itself. Homer, the
simple minded yet loving and dedicated father. Marge, the reluctant and a somewhat naive all star wife and mother, always providing reason and being an anchor for her family. Lisa, the sweet, brilliant little girl who still suffered from the typical problems of an 8 year old girl. Bart, the "good" son with a wild side that always got the best of him, and the ever present Maggie.
Now Homer is just jackass who seems to literally be mentally handicapped. Lisa is an obnoxious left wing activist vegetarian who ends up trying to be the brains and conscience of the entire family. Bart is just a pathetic source of stupid quips, one liners, and blatant, cliche, flamboyant homosexual behavior--along with Milhouse and Smithers, who have gone from humorously questionable to occasionally downright offensive. And Marge has become something along the lines of what Homer was a few years ago before he went completely mentally defective. All of her decency and humanity is gone. She's just another outlet--along with Maggie--for stupid jokes. (Usually
sight gags with Maggie.)
But what pisses me off is hearing people talk about how funny the show is, or how "smart" it still is. Just because they can work in some loose political commentary or pop culture reference in between moments of stupidity, with all of the subtlety of an exploding cigar or hand buzzer.