I believe Seinfeld is the greatest show of all-time—so much so that I gave myself an emotional breakdown from marathoning the whole series in 5 days (http://tedfoxisawesome.com/33-project/the-great-seinfeld-watch/). And I probably think the finale was better than you do. But here are several things I wish they would've done differently.
  1. Use fewer guest characters
    Puffy, Newman, Bania—yes. Every memorable guest character in the history of the show? There were just too many of them.
  2. Make it a regular episode
    Seinfeld was famously "the show about nothing" for good reason: It got lost in the minutia of daily life, gloriously and, increasingly, ludicrously so. The 4 main characters going on trial felt too high concept and not in keeping with what made the show's best episodes great. It could never happen, but how amazing would it have been if something like "The Serenity Now" aired, and that was just it? No acknowledgement it was the end, just walking away. It would've been the ultimate Seinfeld move.
  3. Don't give the 4 their comeuppance
    Jerry, Elaine, George, and Kramer did indeed possess a … uh … questionable code of ethics. But that's what made them so fun to watch: They were the worst parts of all of us come to life, and they were allowed to run rampant in hyper reality. Seeing them get punished, even if they were unrepentant, felt like an indictment of all our shortcomings they represented.