I absolutely love Seinfeld. The ony episode that is a little bit weak (although it is still great relative to most sitcoms) IMO is the original pilot. There was only really one plot line in this one unlike the others that as has been mentioned involved several plot lines that weave about, contain several twists and then resolve towards the end. I became a bit anal a few years ago and sat down with paper and pen and worked through several of the episodes, writing down the plot lines and what happens when. Absolutely magical it really is. I am always surprised when I hear some people say that this has dated because I don't think anything will ever match it. Certainly not sustain such quality over 9 episodes.
It is the ability to make such great comedy out of the simplest of things that made it so special, this is only possible if the characters are absolutely fantastic. As I said in a recent thread George Castanza is a legend.
Watch them from the start slagA, in order, and you'll soon get totally hooked I reckon. It is true to say there are threads that re-appear in much later episodes. Look out for the use of the name 'Art Vandalay' and the references to architects throughout the series!