I got into this show pretty late after long exposure to someone who was REALLY obsessed with it. I guess that was 2002, 2003ish? I was able to find enough showtimes that I rarely bothered to catch up to the new ones intentionally. I just figured I'd get to them eventually. This way I watched every episode up to Season 15 or something. I wouldn't say I stopped because of the decline, because that's not really true and it was more outside factors like getting to university and subsequently watching a lot less TV. However, there is a point in the series where I feel like I'm just watching out of habit.
I definitely agree with Tobes on basically everything. Lisa is the most obvious one for me. She is so intolerant of people she sees as being "wrong", it's as if the series knows that Lisa is a vegetarian but doesn't actually remember the episode where she became one. It's worse that themes like that are also being repeated over and over again, beating a herd of dead horses. I feel like the characters are mere caricatures of themselves now. They've forgotten everything that used to challenge them as legitimate characters before, for the sake of remaining familiar to viewers. Ned Flanders is especially weird because he periodically becomes tolerant and and almost liberal when an episode deems him important, only to snap right back into his mold.
The thing that really gets me, though, is exactly what Kinneas said. It's like the Krusty sketches, where Krusty would make an incoming gag painfully obvious before its execution, like putting a ming vase in front of a door and making a big deal out of it. They're doing that with the show as a whole now. Obviously it's not as blatant as the Krusty sketches but it's there.
obligatory clip I guess
[youtube]1zOuxdRMJME[/youtube]
(gdi it got blocked... but ykwim)