Some of my favorite shows ran their course and their cancellations were timely. But these are shows I loved and believe were cancelled too early and had a lot more good stories to tell...and still do:
Quantum Leap The Byrds of Paradise (if anyone knows this show I'll be shocked) Star Trek: Enterprise FlashForward Seven Days Terra Nova Chuck Revolution
Seven Days was cancelled in the spring of 2001. We all know what happened in September 2001. If the show was still going, there is no way they could have ignored that. They would have had to address what Frank Parker did in relation to the 9/11 attacks. That would have been some dicey writing, and also probably perceived as "too soon." It would have been a Catch 22 for them. Years later though, I wrote a fanfic story about that very issue that I think turned out rather well.
And who doesn't want to know if Sarah Walker got her memory back? :)