Major Offenders: Battlestar Galactica, X-Files, Star Trek: Voyager, Nip/Tuck, Six Feet Under.
Science fiction shows are especially bad about creating a world where no one is ever permanently dead. Cloning, alien abduction and interference by a higher life form are just a few of the ways that shows can bring a deceased character back to life. And then there's the human-looking Cylons on Battlestar Galactica, an entire species that can be shoved out an airlock on a daily basis only to come back once again with perfect hair and slinky dresses.
Why it Works:
Note that this technique often overlaps with the Cliffhanger Cop-Out. Six Feet Under ended Season Two with main character Nate boarding a bus to the afterlife, only to get a poorly-explained resurrection in the Season Three premier. In Nip/Tuck, Dr. Christian Troy is apparently slain by a serial killer in a season finale, gets a funeral the next season, only to have the funeral turn out to be a dream and the serial killer attack having left only a minor cut on his face (you had to be there).
Just as with the Cliffhanger Cop-Outs, this lets the writers have it both ways. They get their dramatic death scene one week, without having to deprive the show of a favorite (that is, ratings-boosting) character. Besides, did we really think Buffy would stay dead all of those times she got killed? Her name is the title of the show.
Why it Shouldn't:
You can see the problem on shows that abuse it, particularly sci-fi or fantasy shows where audiences have gotten used to the idea that anyone can come back (we imagine that very few people actually believed that Kara Thrace died when her ship exploded in a wormhole on Battlestar Galactica).
Viewers saw Jin get exploded on a boat in the finale of last season's Lost, but know that he could be back as a ghost, or through some kind of time travel. Or the healing powers of the island could fix him somehow--hell, it's Lost. There's like two dozen ways he can come back. At this point when somebody gets "killed" we just roll our eyes.
Major Offenders: Almost every single drama or reality show on television.
These come up mostly on shows that hit a dramatic dry spell during the season, when it's up to the promotional "teaser" clips at the end of the episode to promise big things anyway. For instance, look at the last two seasons of The Sopranos.
Once upon a time when Tony Soprano said "We've got to deal with this guy" it meant "The guy" was soon going to be thrown into the ocean with a dozen new holes in his body. What we got in those last two seasons, however, were previews for next week showing Tony saying something like, "We have to deal with Paulie", along with a scene of some dude on a dark street getting shot in the head.
Then the new episode arrived and, no, nothing happens to Paulie. Tony's "dealing" with him winds up being a harsh word while they're eating at a buffet. The dude who got shot turns out to be some nobody we had never seen before.
This is happening more and more with other shows, as the guys in charge of editing the preview for the next week carefully construct an outright lie about what the episode will be about. They'll dub in dialog where it doesn't belong, they'll use misleading edits to make it appear a main character is in danger. Lost took it to a whole new, silly level when the announcer just started lying to our face ("Next week, the entire secret of the island will be revealed!")
Sometimes you'll even spot a scene or a line in the episode itself that seems to have been filmed specifically to include in the preview, as it winds up being totally pointless and out of place in the episode (look for lines of dialog like, "Guys, this could change everything" followed by absolutely nothing changing).
Why it Works:
They know that we tune in for months at a time waiting for some massive shake-up in the plot, either a major character dying or the answer to a lingering secret. But, that important stuff has to be handed out sparingly, you can't have that shit happening in every episode. So, they keep us tuned in by promising next week will be the episode you can't miss.
There seems to be two ways the producers make it up to us when the actual show fails to live up to the promise of the teaser. Either they replace the fake important scene they teased with a real important scene ("OK, so they didn't reveal the whole secret of the island but Locke got shot so that was pretty cool") or, more likely, they give us an episode of filler that ends with another teaser that promises the next episode will be the one we can't miss. Even though we totally could have missed the one we just saw.
Why it Shouldn't:
What could be worse than the kind of "it could only happen during sweeps" ratings stunt that we've all grown to hate? Falsely promising one and not following through. Every week.
Lost and The Sopranos both saw the law of diminishing returns on this, as audiences basically gave up the idea that anything earth-shattering was ever going to happen. By the time it did, most of us had stopped watching.
You can see more of Jonathan Kimak's writing at his blog.