I watched it recently for the first time, and I really don't get why it's so loved. IMDB rates it as the second-best movie of all time, but it seems far worse than that to me. I like most old movies and see their hype, but The Godfather didn't do it for me. What am I missing?
Seinfeld was filmed in front of an audience.
True but it is the same effect as a laugh track. The actors have to pause their delivery to let the audience react so that they are not talking over or getting drowned out by the laughter. There are a few scenes where you can see Jerry and the others almost break character while they are waiting for the audience to finish laughing.
You're missing the point. Back then audiences weren't told when to laugh (that became a thing a bit later). But were more like a live barometer of the script's quality. The recordings of Jerry doing a standup on a bar stage were filmed the same day. They would test material, improvise jokes, actions and lines on the spot and look at the audience reaction, not because they expected a laugh, but because they were getting a laugh. They weren't pausing for the editors to insert a laughtrack, they were pausing because the audience was laughing and their lines would be inaudible on the mics. This allowed them to fine tune the show, then during edit make it as hilarious as possible. It was a thing they actually struggled with the sections filmed out of studio, but they showed them to the audience so they got the context for the set comedies. It was a whole art, and the actors were part of the writer's room. An entirely different vibe from this decade's sitcoms. Modern sitcoms are emulating what they were getting organically during filming.