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<< I'm afraid that's not true. "All in the Family" was much more "revolutionary" than Seinfeld. >>
I'm not talking about revolutionary in terms of subject matter (such as dealing with racism, bigotry and other subjects) which All in the Family dealt with. I'm talking about completely changing the way sitcoms were written at their very core.
Before Seinfeld, everything was plot and story driven. The characters did funny things AROUND the story to support the storyline. Sure, who the characters were had affected what the storyline would be. But the STORY was always central. After Seinfeld, everybody starting shifting gears. Seinfeld was successful with the model of having the characters (and most importantly, their daily, witty dialog) be the central focus of the show, not any set storyline. The dialog determined the course of the show. As the show evolved over the years and built up a history of characters and their personalities and relationships, it too started to lean more towards story-driven comedy instead of character-driven comedy. Unfortunately, the show and its writers weren't as good with that material which is why most people didn't enjoy the show as much during the last couple years. It drifted away from its roots.
After Seinfeld, nearly every sitcom (especially on NBC) tried to mimic the Seinfeld style with the characters just sitting around talking, bouncing their quirky witicisms off one another. Some worked for awhile, but nobody had quite the same flair for it. It looks like that trend is finally dying off some now and we're seeing more traditional story-driven sitcoms again.