Seconding Firefly. I miss that show so much and I wish that it had continued.
Another show that comes to mind is Colony. It ends after the third season, and there are so many story threads left to complete. It was cancelled because of low viewership. It was an original show that blended sci-fi elements with a family drama. It was so good and suspenseful!
Why do the new and bold shows get cancelled, while the boring, repetitive trash gets to stay?