I for one place part of the blame on myself for my disappointment.
My problem is that I have many, many episodes on tape and watch them frequently . . . including the Bubble Boy, for instance. The last thing I wanted to see in the last episode was old episodes rehashed, especially since many of them I have watched recently. Maybe we all just know the show too well (if we didn't, we likely wouldn't spend much time in this newsgroup) and that caused signifigant ire because we longed for new material.
I honestly have great difficulty thinking of an ending that wouldn't have left me disappointed. They should, however, be given credit for jabbing at all of us who tried to predict the ending . . . all the scenarios were hinted at, but none of them happened. I don't remember reading any "prison ending" suppositions . . .
And one more thing - - - did you make the connection in the scene where Jerry and George just left the NBC office, and started to hug, but refrained. Remember Seinfeld and Larry David's motto for the series - - "no hugging, no learning."
I'll have to reserve judgement until after I watch it again in a few days and after the "coffee machine" conversations tomorrow.