I think this one pretty much speaks for itself- many TV shows are canceled before their time, while others (ER, According to Jim) are allowed to run for years and years too long. These are my Top 5 Prematurely-Canceled TV Shows, ranked by considering both quality and lifespan.
5. Star Trek Enterprise - Yes, I know, the show sucked pretty bad. But it was just starting to get good! Season 4, under the leadership of new showrunner Manny Coto, finally turned the show around from unwatchable to genuinely entertaining! Imagine what he could have done with three more years! We may actually have had a Star Trek!
4. Invader Zim - Lost to the fight for a good timeslot, indie comics legend Jhonen Vasquez's Invader Zim was the first and only show on Nickelodeon to drop a six-year-old kid down an elevator shaft. Zim was written by a group of mental patients, making the show's story and dialogue random and often nonsensical, but never lost its focus on being hilarious. Invader Zim was perhaps the first show to be targetted specifically at ADD kids.
3. Arrested Development - Basically wrote the book on what's funny in this decade. Unlike many of the other sitcoms in recent history, the creators of Arrested Development did not assume that its audience was a bunch of drooling morons, but a smart group of viewers who can be trusted to get their ironic, absurdist humor. Unfortunately, the creators of Arrested Development were dead wrong. Though critically-acclaimed with a devoted cult audience, the public at large just didn't get Arrested Development. It was cancelled after two-and-a-half seasons, far too short for a show with such potential. (Had the series been shorter, it would have ranked higher on my list.) The entire show is available on Hulu.com, where it's been welcomed by a whole new audience. I urge you to join it.
2. Clerks: The Animated Series - Kevin Smith's Clerks is a staple in the history of American Indie cinema. The ABC cartoon that it inspired is a staple of DVD collections and not much else. Cancelled after only six episodes were produced and only three aired, Clerks takes the best parts of Clerks and the best parts of Family Guy (meaning the parts that aren't stupid, meaningless frat-boy humor) and combines them to make a whole greater than either of the ingredients. Easily one of my favorite cartoons of all time.
1. Firefly - This is a no-brainer. Joss Whedon's brilliant space western produced only 14 episodes, but managed to inspire an army of fans (called "Browncoats) and influence half-a-dozen other series that followed, including the critically-acclaimed Battlestar Galactica. In only about ten hours of content, including the feature film that followed, Whedon and his collaborating Executive Producer Tim Minear managed to lay the groundwork for a deep, intreguing mythology with simply amazing characters. If I could resurrect one dead series, this is the one.