Too many plotlines
Classic example: Season 2 of Twin Peaks
There are of course exceptions to this rule, but in general, we’d argue that TV shows work best one of two ways: they either have a single strong, recurring narrative thread, or they have a single strong theme that ties each episode together (like, say, Party Down). In the absence of either, writers often tend to compensate with volume, trying to tell too many weak stories instead of one strong one — the result is a surfeit of characters vying for screen time, and an exasperated audience. The two seasons of Twin Peaks illustrate the contrast well; while the show was always character-driven, the first season centered around the mystery of who killed Laura Palmer, with the plot sidelines ultimately tying back into this main narrative. Once the mystery was solved, the writers tried to compensate by developing too many stories, some of them plainly absurd (like Nadine going back to school), and the show completely lost focus. Again, True Blood is showing worrying signs of this — so here’s hoping that godawful little brother really did finally get killed off last episode.
The contrived return of old characters
Classic example: Dallas, again
So ratings are declining and the producers aren’t happy. What about getting some of the audience’s favorite characters back on the show? The good thing about being a writer is that you can explain away pretty much anything — or try to, anyway. And so it is that we’ve had some truly absurd resurrections over the years, like the apparently unkillable Dirty Den in East Enders. The best ever, though, has to be Dallas (again) — the whole ridiculous dream plot line was basically a device to allow the return of a character who’d been killed off a season earlier.
Classic example: The X-Files
Cliffhangers are only dramatic if the audience at least acknowledges the possibility that the impending disaster that’s left looming until next episode (or next season) might actually take place. This is why “Oh God, the main character might die!!!!!” cliffhangers are rarely effective — you know that they won’t. So it was with the “Mulder killed himself!” conclusion to The X-Files‘ fourth season. Since everyone knew that David Duchovny was returning to the show, the question was never whether Mulder would actually be gone — rather, it was how the writers would contrive to bring him back. This isn’t exactly a recipe for dramatic tension.
Classic example: Friends
If nothing else works, you can always fall back on what’s worked in the past — bringing back old characters is, of course, closely related to this idea, but here we’re talking about recycling entire narrative lines. For an example, look no further than Friends — whenever ideas waned, the writers fell back on the Ross-Rachel relationship dynamic (coincidentally, the show’s least interesting idea in the first place)… again, and again, and again. Happily, there’s been little of this in True Blood as yet, although there’s an argument to be made that it’s guilty of the opposite — a kind of ideas-based arms race, whereby each season tries to be more outlandish than the last.
Classic example: Pretty much all of them
Really, has a TV show ever returned stronger and better once a spin-off film has been made? Apart from maybe South Park? If anyone starts talking about a True Blood movie, be very afraid.
Jumping over a shark on waterskis
Classic example: Happy Days
OK, change the channel. It’s all over.