And yes, this is sort of a "jump the shark" thing — except that usually, people say a show jumped the shark when it added a new character, or when there was a brand new direction. At least some of these are cases where a show didn't get rid of a character fast enough, or stuck to its same old direction for too long.
1) Battlestar Galactica
There was a time, not long ago, when BSG was considered the gold standard for science fiction television. Tense, multilayered, complicated, and filled with morally gray characters — it was all the things we always clamor for but seldom get. The show stumbled somewhat in its third season, with the boxing episode and a boring love triangle. But it didn't actually become raw suckage until Starbuck died for no reason, and then came back from the dead for no reason. The writers have basically admitted they had no plan for killing and resurrecting Starbuck — they just thought it would be a cool thing to do, and they would figure out the reasons later. And thus, we ended up with a year of Starbuck Interrupted, constantly shouting "You're going the wrong way!" and playing the piano while talking about her daddy.
The news about MSN bringing this show back sent us on a trip down memory lane yesterday. The truth is, it was a really fresh piece of television, where a lot of the fun came from seeing these disparate characters cross paths and become interconnected. And figuring out the clues, and waiting to see how Hiro became Badass Future Hiro. Unfortunately, as much as we loved this show's characters, the show loved them more — and it couldn't thin the herd. If both Peter and Sylar had stayed dead after the end of season one, there could have been a clean slate and a chance to start over. Sylar could have been gone for a few years, and then brought back in a shocking twist during season five or something. But instead of moving on with a fresh story we got an angsty directionless Peter and the ongoing moral struggles of Sylar, the once-great villain.
3) The X-Files
The X-Files overstayed its welcome, even to its own actors. When David Duchovny left the show as a regular castmember, it killed the relationship dynamic that was the heart of the series. Mulder and Scully’s relationship is what set the tone and the conflict of the skeptic versus the believer — and then later it was them against the crazy government/alien conspiracy. And meanwhile, learning too much about the entire alien conspiracy thing killed the premise of the show. We didn’t need to believe anymore. We knew what was out there, and it was a grinding government conspiracy with large plot holes and unresolved issues.
It's hard to say anything too mean about Fringe — it was a show that swam against the tide, having a scientist antihero as its central character, at a time when you can only have science fiction if there are no scientists.(The decision to make FlashForward a show about FBI agents instead of physicists pretty much wrecked that show from the get-go, but that's a rant for another day.) Fringe had a pretty solid first season, and then mostly brilliant second and third seasons — and then it wrapped up the main arc of the series, about the clash between parallel universes, prematurely. No more epic clash between Walter and Walternate, no more Peter-Olivia-Olivia love triangle. And no more about the consequences of Walter breaking a universe. The show still had a few good episodes after that, and the final season found an interesting new direction, but it never regained the heights of seasons two and three.
5) Blake's 7
Blake's 7 is sort of a weird case, because it did manage to have a fantastic final episode that sort of redeems the awful mess that preceded it. But in a nutshell, this show completely tanked when it became the Avon Show. Blake's 7 started out as a brilliant space opera about criminals and revolutionaries joining forces to fight an oppressive space empire, the Federation. There's virtually no way the Syfy remake can live up to the original Blake's 7 at its best. Avon was the best character on the show, a sardonic, misanthropic computer nerd who was always ready to double-cross Blake but also kind of loved him. (He should be played by Mark Sheppard in the new version, if Syfy is smart.) But after most of the other original cast members left, the show leaned on Avon more and more, and Paul Darrow's performance got more and more operatic. Until you get whole episodes where it's just Paul Darrow chewing the scenery while cheap cardboard sets burst into flames.
This show was always about the relationships, more than about the plot mechanics or big mysteries. My theory is always that when people say "I didn't get enough answers from Lost," they really mean, "I didn't get enough resolution with the characters, where all of this wound up meaning something to them." And one of the main relationships in the show was between Jack and Locke — which carried thematic as well as personal weight. They were the Mulder and Scully of Lost. We spent a year wondering who was in the coffin, and when it was revealed to be Locke, the show lost some of its mojo. We got to see Jack come over to something like Locke's point of view, but we didn't get to see the two of them talk about it. Especially since other relationships, like the Jack-Kate-Sawyer triangle, also ran out of juice, the loss of the Jack-Locke relationship was a major blow.
7) Lois & Clark: The New Adventures of Superman
This show had some sparkle at first, with its witty character banter and fresh look at Superman. You have to give this show some props for helping to bring superheroes back to television, and for making Lois more of a protagonist instead of just Superman's love interest. But the show suffered from the fatal flaw of being built on the "will they-won’t they" dynamic. This tension had to be resolved eventually, and once Clark and Lois ended up together, the show pretty much ran out of steam. Sure there were attempts to spice things up like a frog-eating Lois clone, but it wasn’t going ever recapture the initial romantic conflict.
Sliders started off as a show about a group of people lost in alternate timelines trying to make their way back home but taking the time to save those other realities with their weirdly useful skillsets. Things started going bad around the third season when the network relocated the show and started making script demands, and then fan-favorite actor John Rhys-Davies quit the show. But the show didn't fully go off the rails, in terms of its storyline, until it made the Kromaggs, formerly a villain of the week, into a big bad. The show went from exploring alternate timelines to an us-versus-them scenario.
9) Twin Peaks
Twin Peaks was atmospheric, creepy and weird as hell. Though it was strange and confusing it was at its center a murder mystery, a mystery that led to wild fan speculation and half-baked water cooler theories. When the identity of Laura’s murderer was revealed, there wasn’t anywhere else to go. As Eli Roth said the other day while talking about Hemlock Grove, "As much as I think Twin Peaks is my favorite show of all time, you can tell they never intended to solve Laura Palmer’s murder. I’ve talked about this to David Lynch and the network made him do it. And after that happened, they had to figure out a new mythology."
Want to see what it would be like if someone wrote an authorized Harry Potter sequel, picking up after Harry defeats Voldemort? All you have to do is watch Supernatural seasons six, seven and eight. Supernatural had a nearly perfect ending — seriously, Battlestar Galactica and Lost only wish they could have ended with an episode that ties up all the themes and character stuff as brilliantly as Supernatural's fifth season finale. Creator Eric Kripke fully intended for that to be the last word on Supernatural, but The CW apparently filled his swimming pool with bearer bonds, because the show came back for three seasons (and counting) of aftermath. Part of what makes stories great is that they end — and prolonging a story past its ending is like following your friends home and watching them sleep. All night.