I tape every Seinfeld episode shown here in Seattle, and I have noticed that one channel I tape has been showing the 1st season, and the other channel has been showing much later episodes. Every night my wife and I watch 1-3 episodes, and it is really funny to watch the old ones back to back with a newer "in-their-prime" episodes. Some things I have noticed:
Jerry is a very consistent character, acting-wise. For all the slamming he took as a pure actor on that series, he portrays "himself" almost identically from season to season (unlike how Elaine goes through a few striking changes). Elaine, George, and Kramer all have vastly different personas in the later seasons, which probably has more to do with the writing then the acting... George is much more whinier and neurotic in the early seasons. When did George switch over to the more "confident" loser/liar we are accustomed to?