The plotting of most episodes was phenomenal, that's what I liked about it. Each character would start off with their own little story or issue, and by the end of the episode they'd all come together for a hilarious payoff. The episode where George has been telling a chick he's a marine biologist and at the end has to go yank a golf ball out of a whale's blow hole that Kramer had hit in to it is a perfect example.
I agree that Seinfeld himself isn't very funny, either in the show or in stand-up. But the show itself is hilarious and pretty timeless (a lot of sitcoms age very badly). The only exception to Seinfeld not being that funny was the episode where he and Kramer trade apartments, and start becoming each other. Seinfeld as Kramer was pretty fucking funny.