By this point the series had settled into a too-comfortable format that this season exemplifies.
1. Hilarious first act that is soon left behind by the actual plot.
2. So-so second act that introduces the gratuitous guest voice and expands on the plot.
3. Hit or miss third act that either veers completely away from the general plot or takes the premise to an absurd level. Depending on the execution and one's personal tastes, this makes the episode either decent or awful.
Individual episodes deviate from this at times, but for the most part they stick fairly religiously to it. Did I say the guest voices are gratuitous? The stories should not be notable mainly because of the guest voice. This season leads off with The Who episode (which the commentary reveals doesn't feature the entire group, even as it existed then), a symbol of what's to come.
That said, I still give this 4 stars because of the mostly great first acts, which this period of the show elevates to an art form. The best ones have almost nothing to do with the direction of the rest of the episode but are loaded with great gags, like the trip to the Festival of Books in "Insane Clown Poppy."
Some of the third acts are brilliant, too, like The Prisoner homage in "The Computer Wore Menace Shoes," or the mob family gathering Homer & Krusty invade in "Insane Clown Poppy."
What's largely missing is the emotional core that was so memorable in the first several seasons. By this point many of the writers, directors, and producers who created and cared about these characters have all have long since moved on to other things. The commentaries make pretty clear that the 12th season staff just like making Homer appear as dumb and depraved as possible. "Homer vs. Dignity" is a particular low point featuring Homer doing baby talk in a diaper and getting raped by a panda. When genuine emotional substance is called for, such as Krusty trying to establish a relationship with his previously unknown daughter in "Insane Clown Poppy," no feeling comes through at all. (Compare it to "Like Father, Like Clown," where Krusty reconciles with his dad, or the end of "Lisa's Substitute" when Homer gets Lisa to laugh with him again.) In this specific case, part of the problem is Drew Barrymore's generic voice work completely fails to convey the emotion that's animated for her. In general, though, it reveals that the producers generally seemed to coast through the year.
Still, there are some highlight episodes, like "Trilogy of Error," and "HOMR," and the annual "Treehouse of Horror" is a little better than some previous years' Halloween eps. Even the lesser episodes have great moments. The animators' commentaries are interesting as they discuss and illustrate on screen certain aspects of drawing the show. The Comic Book Guy's highlight reel is fun to watch. There is still plenty to enjoy, even if you maybe won't enjoy it quite as often as the earlier seasons.