That's the appeal about the Simpsons, is that there are so many subtle things that it takes several viewings before you notice things. I continually watch the Simpsons in syndication, even though i've seen some episodes several times.
Family Guy is the first (and only so far) TV show box set I've purchased, and will get season 3 (last before hiatus) and beyond eventually on DVD. The full-length movie will be coming soon (
Stewie Griffin: The Untold Story!).
American Dad is getting better. But, quite honestly, I don't see it lasting.
I find myself avoiding primetime TV for syndicated reruns. Sitcoms of the late 80s and 90s I think are far better than the crap that's out there today (exception is Sunday Night on Fox, Arrested Development and Malcolm in the Middle, but even Malcolm isn't as good as it once was). However, I did start watching Everybody Loves Raymond in syndication, and it's freakin hilarious.