Always Murder: Enforced by the show's format, with three notable exceptions.
"Just Another Fish Story": Self-defense.
"To The Last Will I Grapple With Thee": Suicide made to look like murder to implicate someone else.
"The Christmas Mystery": The intended murder victim is shot in the head, but it is later announced that she has recovered.
Amateur Sleuth: Jessica has no formal law enforcement or investigation training, and never acquires any throughout the series. Although, once, when a witness is unwilling to be questioned with Jessica present, the sheriff says he'll deputize her if necessary.
One murder victim turns 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.
The killer in the first episode is being blackmailed over a crime for which he was framed originally, and kills his blackmailer.
Asshole Victim: Almost every murder victim in the history of the show likely qualifies, with a few exceptions. There's never a case where the murderer is some brutal and heartless killer; he or she is almost always someone who the victim has wronged in some way, even if the killer themselves turns out to be a jerk.
Cabot Cove's first sheriff is Amos Tupper, an honest cop and a close friend of Jessica. He retires after the fourth season.
Tupper is replaced by Sheriff Mort Metzger, a former NYPD detective who takes the job in the mistaken belief that the town is a peaceful place.
Chuck Cunningham Syndrome: Ethan Cragg (Claude Akins), who appears 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. In a later episode, Seth expresses concern that she's investing too much of her time in her books and missing out on life.
Crossover: With Magnum, P.I.. The crossover episode is a two parter, with the first part on Magnum, P.I. and the second part on the subsequent episode of Murder, She Wrote.
Doom Magnet: Is Jessica Fletcher visiting your town or home? One of three things is going to happen. You'll either be killed, accused of killing, or be revealed as a killer. Being accused is your best bet, as Jessica will invariably prove you innocent when she reveals the real killer.
Lampshaded more than once.
Dude Magnet: Jessica gets a lot of attention from older gentlemen and in one instance a college-age Cake Eater flat-out propositions her.
Eagle Eye Detection: Jessica's ability to pay attention to minute details is usually what reveals the killer. In one example, Jessica notices that one character has been hiding his hand in his pocket since the murder because a dog bit him after he committed it.
Friend on the Force: Jessica acquires a large number of friendly detectives as the seasons go on, not to mention her friendships with the Cabot Cove sheriffs.
Fully Absorbed Finale: The "bookend episode" The Grand Old Lady, was actually an unfilmed series finale script from a previously-produced and cut short Ellery Queen series, albeit with the characters slightly changed to dance around rights issues. Both series were produced by Levinson and Link, and they apparently decided not to let a good script go to waste.
Identical Grandson: Jessica has a British cousin named Emma, also played by Angela Lansbury. In the episode "Runs in the Family", only Emma was featured, with Jessica not appearing at all, and Emma proved to be a rather good sleuth herself.
Instrument of Murder: "Death to a Jazz Beat", offered an inversion; a jazz clarinetist who was planning on firing his back-up band before signing a huge recording contract was murdered by a poisoned clarinet reed.
Just Between You and Me: The killer always confesses to Jessica in the belief that they are alone, just before trying to kill her.
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.
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 parody of the series is named "Murder, She Hopes." Jessica is overjoyed every time she learns that a new murder has taken place.
She is never said to have been a native of Cabot Cove; her husband was career Air Force and they went there after he retired. She taught H.S. English; he probably bought a boat and puttered around on it.
Obfuscating Stupidity: Jessica doesn't hesitate to play up her "little old lady" image to get information out of people. In a few instances she even played a drunken floozy with surprising skill, which may be justified since Jessica has a history of theater in her background.
Oireland: "The Celtic Riddle", "Nan's Ghost", "A Killing in Cork", "Another Killing in Cork", "To the Last Will I Grapple With Thee"(set in New York but involving an Irish blood feud)...
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.
Sympathetic Murderer: Frequently killing the Asshole Victim. Sometimes it's self-defense or accidental, in one case of the latter Jessica agreed to keep quiet about solving the murder because revealing the "killers" would have caused harm to an innocent person. Some of the murderers lose their sympathy when they try to frame someone else or try to kill Jessica to cover it up, though.
One of the show's final episodes, "Murder Among Friends", features a murder taking place among the cast and crew of a show, titled "Buds", about six young people trying to make it in the big city. Sound familiar? The show got slotted against Friends as part of a plan to kill it off.
The final episode, "Death by Demographics", similarly knocks on the reason for the show being death-slotted: its lack of appeal to the 18-39 demographic. The episode itself is about a radio station manager who fires every employee over a certain age and changes the station's classical music format to more contemporary hard rock.
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 final movie shows that Jessica has even begun using a laptop.
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 Miss Marple film Lansbury appeared in.
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 Jonathan 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.
Leslie Nielsen appeared in the first season as the captain of a cruise ship when Jessica gets involved with a murder aboard the ship. He turned up again in season three as an old admirer of Jessica's who spots her on the street, plants a huge kiss on her lips then shortly after gets accused of murder.