Classic style:
[When Bart quits, because he didn't get good at the guitar right away] Homer: Son! If something's hard, it's not worth doing. You just put that guitar back in the closet with your karate outfit and your unicycle, and let's go watch TV.
{After any particularly violent (early) Itchy & Scratchy cartoon] Bart, Lisa, and Homer laugh really hard
New style:
[After practicing the first line of their hip-hop style song] Nelson: Isn't this song a little boastful?
[After seeing a gratuitously cast Bumblebee Man in "Dawson's Creek"] Lisa: I'm all for ethnic diversity, but this is just pandering.
~~~
Classic social commentary on the Simpsons was hidden, and sarcastic. It's satire when the family laughs at an extremely violent cartoon. "Tom & Jerry" and other cartoons from the past seemed to perpetuate this idea, and "Itchy & Scratchy" exaggerated it greatly. The family laughing at such violence is the satire, because it sarcastically suggests that extreme violence is funny. This also mocks all similar shows, however relatively mild, that came before it. In the world of the Simpsons, just an exaggeration of our own, extreme violence is funny -- and it's normal.
Social commentary in the newest episodes, it seems, comes pre-packaged, without the wrappings of delicious sarcasm to decipher. Something from our lives is demonstrated, then one of the characters makes a comment. There is no sarcasm, and any mockery of the subject is dulled. We aren't the observers, anymore; we're not seeing the commentary. It's being given to us, as if we wouldn't get the joke otherwise.
Now, Itchy & Scratchy have made their point a number of times on The Simpsons. Perhaps that's why we rarely see them anymore; the "extreme violence is funny" joke has been done many times already. But, as long as there is life on planet Earth, there will be things to mock about us. There will be observations to make, and exaggerate.
In Simpsons satire, the characters shouldn't "make" the commentary; they should "be" the commentary. When the characters themselves "are" the commentary, we see their exaggerated situations as social commentary. When characters "make" the commentary themselves, they pull back into our world and the irony and sarcasm are lost.
Bringing back characters "being, not making" commentary would add the level of irony and humor back to the current show that is largely missing.