hmmm... well, what I like about the Simpsons (and they are in a class of their own in my book) is that it takes pertinent and sometimes delicate issues and addresses them in sometimes over-the-top and comical ways that actually get some interesting points across. And the overall affect, while sometimes thought-provoking to an extent, is quite light-hearted.
As far as the family guy goes, some of it deals with interesting things related to family dynamics, but some of the humor seems to be there strictly to push the artistic envelope, which is part of its charm to me. However, occasionally I find it mildly distasteful and I am somewhat putoff. Same with American Dad (is that the name ?). And ultimately, I think Family Guy is successful because the Simpsons paved the way (as with the other grown-up cartoons that have followed the act).
I have occasionally gotten bored of the Simpsons and have stopped watching it religiously, but I have never really felt put off by the content in the show, which I think shows class on their part. Plus, they have great music !!
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There is more I could say about the Simpsons, but, nah.
Karli