I've loved The Simpsons for many years. I've bought every DVD box set and continue to watch the show on television, and despite the decline in quality, The Simpsons is still one of the best reasons to own a TV. And as a superfan, I believe that this, the fifteenth season, was where OFF started to lose it.
Although there are a lot of good episodes here, they never really get any better than just "good". None of them are as bad as I remember, although like the season before it, perhaps mainly because they aren't as Lackluster (Video) as they are now.
The decline begins with the episode "Catch 'Em If You Can". I never liked this episode originally, and having rewatched it, my opinion hasn't changed. In this episode, Homer and Marge ditch the kids to spend some intimate vacation time together. Fair enough. But Marge, particularly, in this episode behaves extremely out of character. Portrayed as a mother desperate to escape her children, it totally destroys her loving nature. What happened to the woman who couldn't bare to be separated from her children in the seventh season episode "Home Sweet Homediddly-Dum-Doodily"? In this episode, Marge also uses language which is usually absent from her vocabulary such as referring to sex as "sex", whereas in earlier episodes (seasons) she would have normally used a euphemism such as "snuggle". Maybe I'm looking into it too much, but this isn't how classic Marge would act. It's almost as if they were trying to turn her into a poor man's Lois Griffin at this point. On a related note, in the following episode she refers to contestants on a Bachelor-type TV show as "skanks".
What you're getting with the fifteenth season is the beginning of the end. A strong start for a season, and a series that perhaps in some opinions should be put out to pasture, and a mediocre end.
It's The Simpsons, Jim, but not as we know it.