Two notable exceptions; "Just Another Fish Story"(self-defense), and "To The Last Will I Grapple With Thee"(suicide made to look like murder to implicate someone else).
Anti-Villain: One murder victim turned out to be blackmailing the more prominent men of her small town. But then it's implied she was using the money to anonymously support charities for orphans and widows.
Chuck Cunningham Syndrome: Ethan Cragg (Claude Akins), who appeared often enough in the first season to be considered a recurring character, disappears without explanation at the start of season two, his role as crusty Cabot Cove resident filled by Dr. Seth Hazlitt.
Creator Breakdown: Reversed, in-universe. The reason Jessica started writing her mysteries in the first place was to give herself an outlet to work through her grief over her husband's death.
Crossover: With Magnum, P.I.. The crossover episode was a two parter, with the first part on Magnum, P.I. and the second part was continued on the subsequent episode of Murder, She Wrote.
Law of Inverse Fertility: Jessica and her husband Frank were childless; in the pilot, when speaking to a potential new love interest (who later turns out to be the killer), she explains that "We were never blessed in that way," suggesting that they wanted children but couldn't have them for whatever reason.
Magazine Decay: In-Universe example: a publishing magnate buys up a literary magazine called Literary Lines and adds Maxim-esque pictorals of bikini-clad ladies. This doesn't sit well with Jessica, who is under contract to have her first short story published in the magazine.
Murder Mystery Magnet: People drop dead around Jessica everywhere she goes. Everywhere. It's creepy.
It's been calculated that Cabot Cove has a murder rate of 86 per 1000; by comparison, the most murderous city in the world, Caracas, has a murder rate of 1.1 per 1000. That's not counting the murders that happen outside Cabot Cove...
In some years, more people were murdered in Cabot Cove on the show than were murdered in the entire state of Maine in Real Life.
Lampshaded in one episode when another character tells Jessica, "If murder were a disease, you'd be contagious."
Lampshaded again by Sheriff Metzger, a former New York cop, who after a year as the sheriff of Cabot Cove, asks Jessica, "Just what the hell's wrong with this town?"
The MAD Magazine parody of the series is named "Murder, She Hopes." Jessica is overjoyed every time she learns that a new murder has taken place.
Put on a Bus: The original sheriff retired when Tom Bosley left the show.
Nephewism: Jessica had no children, but many nieces and nephews. Grady Fletcher was the only repeater among them.
In one episode, it's explained that his parents died when he was little and Jessica and Frank raised him, thus giving him a reason to be a repeater nephew.
Jessica's other repeating relative was a niece and nephew in law played by Genie Francis and Jeff Conaway who had a continuing thread about wanting to be in the entertainment business.
Poorly Disguised Pilot: In everything but intent. When Angela Lansbury started to tire of the pace of a weekly network show, a strategy was devised that would allow the network to do a full season without Lansbury having to do a full season. Slightly more than half of the episodes of the season would be full adventures of Jessica Fletcher. The remainder would be Poorly Disguised Pilots, for which Lansbury, as Fletcher, would film bookend sequences, explaining the new character we'd be seeing for the next hour — sometimes "real-world" acquaintances of Fletcher, sometimes Jessica's own fictional characters. They weren't really intended to Spin-off any of the characters (although if any were exceptionally successful, why not?)
Script Swap: Done in one episode with an aging actor whose memory is so bad he has to rely on the teleprompter. While this looks like an Engineered Public Confession, it is actually a ploy on Jessica Fletcher's part to trick the real killer into exposing themselves.
Sherlock Holmes: In the pilot, a man dressed as The Great Detective is the murder victim.
"Prediction: Murder" has a housekeeper character named Greta Olsson.
Strictly Formula: Mostly played stright, though it should be noted that this was subverted from time to time. Earlier seasons, strangely enough, played with the formula more than later ones.
Technology Marches On: During the run of the show Jessica goes from typing manuscripts on a typewriter to using a computer as reflected in updated opening credits.
"The Murder of Sherlock Holmes" becomes an Unintentional Period Piece thanks to this trope. When Jessica encounters the two young thugs in the alley, she threatens to call the police, but one of the thugs points out that it would be impossible for her to do so because she couldn't possibly have a phone in her purse.
Trans Atlantic Equivalent: More than a few people have mentioned that Jessica Fletcher could be considered an American Miss Marple. Especially hilarious, as Angela Lansbury has played both. In fact, the opening of "The Murder of Sherlock Holmes" is practically a direct lift from the opening of The Mirror Crack'd, the film Lansbury appeared in.
What Could Have Been: Jean Stapleton, better known asEdith Bunker, was first offered the role of Jessica Fletcher. Stapleton passed - ironically, given that she's on record as being unhappy with her Type Casting as Edith, and this show likely would have rescued her from that fate. In any event, the producers then offered the role to their second choice, Angela Lansbury, who gratefully accepted.
Why Don't You Just Shoot Him?: Excusable early in the series when she was a little-known author and people underestimated her, but by later series people are well-aware of both her books and her reputation as an Amateur Sleuth. For some reason, criminals, even those who have no problem murdering multiple victims in the episode, rarely consider it prudent to try to bump her off as well.
Though there are still a number of people who are willing to try.
This even happens with recurring guest stars. Ron Masak, before joining the cast as Sheriff Mort Metzger, appeared in two earlier episodes (the first playing a cop in New York!).
William Windom played one of the guilty party in "Funeral at Fifty-Mile" before appearing as Dr. Seth Hazlitt in the second season.
Madlyn Rhue, before playing Cabot Cove's librarian in later seasons, appeared earlier as a victim's widow in "Seal of the Confessional".
Another interesting example is "Murder on Madison Avenue": Firstly, after having played Johnathan Quayle Higgins in the aforementioned Cross Over with Magnum, P.I., John Hillerman appears in this episode as a completely different character, to the likely confusion of longtime viewers. Secondly, Barbara Babcock's character (her fourth on MSW, fifth if you count The Law and Harry McGraw) is murdered by her assistant (played by Hallie Foote), but in an interesting possible Casting Gag, the next season episode "For Whom the Ball Tolls" sees Babcock and Foote together again (playing completely different characters, of course), working together on a historic preservation commitee, as if nothing had ever happened.