I thought everyone knew that it wasn't a show about nothing? Jerry Seinfeld addressed this himself on the Reddit AMA that he did recently.
I suppose the joke is that what most people perceive as "nothing" is actually "life". Waiting for things to happen, making plans and watching them go awry, irritation with/ignorance of/deliberate contravention of societal expectations and norms, etc. Obviously it's a show and unrealistic/wacky things happen, but I think it is difficult it was about nothing ...or if it is about nothing, so is every other sitcom (and a lot of other stuff besides).
Kramer to George: "You're wasting your life."
George to Kramer: "What you call wasting I call living!"