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1. It's straightforward. You know, intro, characters, murder, clue collecting, murderer revealed. Although it did get a tad formulaic in its later years, the show served up some good mysteries that made it a Sunday night staple on CBS for much of its 12 year run.
2. The guest stars. Sure, many of today's stars were on the show before they were really famous, but the show's well-known for featuring stars from Hollywood's Golden Age, such as (in no particular order): Milton Berle, Joey Bishop, Arthur Hill, Ned Beatty, Janet Leigh, Leslie Nielsen, Martin Balsam, Ernest Borgnine, Buddy Hackett, Virginia Mayo, Lynn Redgrave, the list goes on and on...
3. The guys behind it. Richard Levinson and William Link, best known as the creators of Columbo, teamed up with Peter Fischer to rework some of the elements from their short-lived Ellery Queen series into a much more successful premise.
4. Cabot Cove, Maine. A cozy, coastal New England town with a staggering amount of murders per year. In fact, the constant appearance of dead bodies in remote locations coined the term "Cabot Cove syndrome".
5. Angela Lansbury, of course. Charming, likeable, and talented enough to play the down-to-earth retired English teacher-turned bestselling mystery writer, Lansbury plays Jessica Fletcher better than anyone could imagine with the actresses who were offered the part initally (could you imagine Jean Stapleton headlining this show?). Check out early episodes when Jessica goes "undercover" or in disguise during a case to see how good of an actress she is.