Friends ended predictably, and Seinfeld ended somewhat disappointingly for such a great sitcom. I can remember both finales (because they still air frequently).
Some series never even get an ending due to programming decisions. I don't think CSI Miami, which has been pulled after 10 years, had a conclusion. Or did it?
Actually, I liked what could have been the finale for CSI New York. I think the series has been renewed, but it was a close call. And last season's finale, with Mack Taylor shot in a convenience store holdup and fighting for his life, mirrored the series itself, which was teetering on the edge of cancellation.