Have we seen this plot before? yes we have albeit not in a place this great looking and thats part of the charm of this fantastically addictive series. So many shows like this take place in a normal setting. This one defies the rules and showcases it in a very rich, very snobby like area full of interesting people.
There are several reasons why this show is great and successful
1. Seth Cohen is the most interesting character the show has to offer. Ya know it all started out with ryan who is the main character but after about 5 episodes it was clear who the real star is. No other character is as fleshed out and real as he is. and funny with his wisecracking demeanor. Seth has actually changed and become more real as the season went on. Most everyone else has stayed the same. Like ryan who hasn't showed any other kind of side or major insecurities, and he's still a hot head.
2. It doesn't try and shove morals down your throat. Just about every show in this vein out there tries to make every episode a "dont do this" episode, and its very refreshing. They present each episode as a day in their life and thats it.
3. Finally parents get their due. Seth's parents are not some stupid "john hughes" parents with little or no screen time and who's pets have bigger brains that they do. Instead they have a decent amount of screen time, fight, make up, and actually have a life that we see.
Is this show perfect . . . . no that would be unfair to say it is or expect it to be. Its fun, interesting, and is the most refreshing addition to the teen genre by far.