There's a list floating around the various TV and SF circles that lists the author's idea of the top ten SF series that were canceled before their time should have been up. It's been ridiculed for topping with the original series of Star Trek and leaving off the truly worst network blunder, Firefly. I won't bore you with the original list. It's just plain wrong.
Here's the right list, counting down from 10 to 1:
 Max Headroom: Played by crazy Canuck by choice Matt Frewer (most recently in Eureka, which has thankfully NOT been canceled!), the TV construct was waaaaay ahead of its times. Figures it came from Britain to be buried in America. Canceling the show meant no more Amanda Pays until The Flash. And we see way too little of Mrs. Corbin Bernsen as it is. If this show were invented today, it would be a hit with the attention-deficit inflicted audience. Unfortunately, trying a revival wouldn't work unless it is dark and moody, as in the Battlestar Galactica way. And that wouldn't work with this show.
 The Flash: Did I mention Amanda Pays was in this show? Oh well. John Wesley Shipp was a credible Barry Allen and the show was moving right of centre when it came to camp. Had it completed the move towards the kind of territory Smallville mined later, it might have lasted a full contract commitment. With super-speed being the easiest of the super-powers to mimic on TV, it sure wasn't an excessive SFX budget.
 Dead Like Me: At least a movie's being made to salve the missing third season wounds. What does it say about me that I prefer to moan over the early demise of dead George, played by Ellen Muth, than the absence of Tru Calling, which featured Eliza Dusku as a life-saver. Bluntly, I prefer the shenanigans of Mandy Patinkin to those of Jason Priestly, I guess. There was just something deadpan and funny about Muth and Patinkin. We'll see after the movie. If it wraps it all up, then just change this list to include Tru Calling.
 Journeyman: This show might have tanked after another nine or so episodes. The potential to flop was always there, what with its inevitable comparisons to Quantum Leap. And it didn't help that supporting star Moon Bloodgood was doing a turn not too different from her role on the tanked Daybreak. But if it HAD reached its potential, NBC was getting rid of a tentpeg show. Stuck traveling through time at the behest of some unknown agency, Kevin McKidd's Dan Vasser lived as much in now as then. That was it's leg up over Quantum Leap. And we all miss not seeing Gretchen Egloff weekly.
 Now And Again: John Goodman dies of a gigantic heart attack. At least most him. His brain survives in the form of genetically enhanced host Eric Close, who then struggles with trying to keep an acceptable distance from Margaret Colin, Goodman's wife. Keeping a leash on government black op Close is Dennis Haysbert, later to play Commander-in-Chief in 24 and a team commander on The Unit. Close, of course, moved onto Without a Trace. This was top-notch casting. And Colin is still a middle-aged man's idea of a hot mama. And daughter Heather Mattarazzo played smart, if not cheerleader cute. It's the least SF of all the shows on the list. But it had a real heart. We will never know if Colin's character figured out her husband survived, sort of.
 Crusade: Boy was there a lot of material to come with Babylon 5 spin-off Crusade. Gary Cole's new spaceship had a mission to extend the life of the Babylon 5 universe and it was a rich one. What with Tracy Scoggins in tow more often and the enigmatic Peter Woodward, a technomage, to create new and interesting directions to go, this was a good idea. Unfortunately, the early bugs were too many to let the networks let it find its way. Given a full first year, Crusade would have matched the length of its parent, I am sure.
 The 4400: I hate, I hate, I hate shows that end on cliff-hangers (see #3). That there is no closure to four years of The 4400 is criminal! It wasn't that the show mined completely new SF themes. Robert Silverberg's Riverworld was surely a template for the starting point of this series. A bunch of Earthlings from disparate times get kidnapped. Instead of ending up on Riverworld, they are deposited back on Earth in the 21st century. Some come back ... enhanced. Handling that ... and them, falls to a pair of newly-minted agents of the National Threat Assessment Command, played by Joel Gretsch and Jacqueline McKenzie. Right from the start, it was hard not to feel for some of the 4400, including characters played by Mahershalalhashbaz Ali and Laura Allen. And I NEVER got over Conchita Campbell's creepy little Maia. No resolution really stinks, though.
 John Doe: Possibly the best cliff-hanger scene of all time NEVER GETS RESOLVED after John Doe was canceled without a second season to explain WHY the friendly bartender, Digger, is the man climbing up the ladder to escape John Doe . He's apparently the leader of the conspiracy John Doe's been fighting! Dominic Purcell as Doe and William Forsythe as Digger had 22 episodes to grow what looked like a pretty solid friendship. Why? What? Will there ever be a when? The enigmatic human computer with no memory should have had at least as long as some second-rate fare like The Pretender had.
 Space: Above and Beyond: The one and only season of this show stands up decades later as watchable, believable science fiction. It was Battlestar Galactica before there was a Galactica revival. It was warfare at its dirty and grimiest best. It showed how camaraderie and sheer will often are what wins battles, not backseat politician-generals who have an ease to spending lives and no knowledge on how to survive and thrive. James Morrison's squad of fightin' fighter pilots, led by Kristin Cloke, were all memorable right from the first episode. So were the enemy chigs. Frustratingly brief glimpses of potential rapprochement in the final few episodes suggested an uneasy peace was possible. Definitely a second season would have entertained and informed.
 Firefly: Obviously! The show sustained a good movie, albeit one that ended with fewer characters than it started with. Nathan Filion proved quite capable at both action AND the fast patter written by Joss Whedon. His crew that included second in command Gina Torres and her pilot husband Alan Tudyk, mechanic Jewel Staite and heavy-lifter/comedic relief Adam Baldwin, were uniquely entertaining in many ways. The Firefly carried passengers, not the least of which was Morena Baccarin, worthy of watching read a telephone book for 45 minutes all by herself. A well-realized Whedonverse, replete with bad guys after two other passengers on the Firefly, and solid SF extensions to what an intergalactic civilization would be like (Think ante-bellum South, with heavy Chinese influence). It was ultimately too literate for the masses who see SF on a show's description and flip the clicker to see if what they are pitching on the shopping channel might be worth buying.
Hmm, two of the shows have movies or are in the midst of making one. The Flash has popped up on Smallville.
There IS life after cancellation. Just not enough of one.