So when did the Simpsons start to suck?

I actually had to consider what the last good Simpsons season was recently because of some great "Buy 1, get 1" deals. After scrolling through season episode descriptions on Wikipedia, I got up to Season 9. Season 9 has some hit or miss moments, but Season 10 has only 1 or 2 episodes that I liked (the Japan episode, that's mostly it).

As others have said, once Homer became a jerk instead of just a buffoon that loved his family the show lost its charm. Season 9 was also the last season that Bill Oakley and Josh Weinstein worked on, so I guess they were the key. In addition, I've never forgiven the writers for killing off Maude Flanders. That event was just plain cruel on the characters and the viewers. That episode happened in season 11, by the way.

Regarding the seasons airing now, they've resorted to ripping off Family Guy style humor and inject a HUGE amount of political commentary (ham-handed, not subtle) that just isn't funny.

Rule of thumb is to watch up to Season 9 of The Simpsons and then just move onto Futurama, which kept the heart and soul of storytelling with witty writing.

Jam it back in, in the dark.