I've said it before, and I'll say it again: 7 seasons is about the maximum length for me. I think if a show goes much further beyond that, it is almost impossible for the writers to keep up the quality. In short, the show goes stale. DS9 ended at just about the right time. I'd rather 7 quality seasons than 20 progressively more mediocre ones. That's where Trek shows, Buffy and Angel, Bab 5, etc, got it right.
The Simpsons was great til about season 9 or 10, then it just went to pot. I can barely even watch the dire new episodes.
L&O I watch casually. There's no way I could go out of my way to see every single episode. It's just too formulaic.
Same with CSI.
Frasier was a great show, but I think even they were pushing it when they went for 11 seasons.