One of the good / interesting things about Seinfeld was that none of the characters ever really had an arc (the only exception I can think of would be George's engagement, his various jobs and stretches of unemployment, and the show he and Jerry were going to create) and the stories were mostly, for lack of a better word, simple. Most episodes were pretty stand-alone and, generally, nothing particularly important happened to the characters. I think that really helped the show stay fresh for a long time, since they did not have to contend with the pitfall of writing themselves into corners.
Take Scrubs for example. I liked the first few seasons, but by season four, so much shit had happened with the characters and their relationships with each other that the writers had to take into account. Characters being married, having kids, going through messy breaks up, promotions, deaths, etc. Eventually that really starts to limit where you can take a character. With Seinfeld, they avoided this pretty well and so still had a lot of freedom with the characters late into the show's run.
I say this, of course, as someone who was never that into the show. I always liked George and Kramer. I did not care for Jerry, though.