These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ensemble Darkhorse: Andy and his family were meant to be the focus, but Barney stole the show.
Idiot Plot: Numerous episodes would have been resolved in under a minute had someone decided to tell a slightly uncomfortable truth rather than try to spare someone's feelings. "Dinner at Eight" is probably the worst offender, when Andy was 'forced' to eat three spaghetti dinners in one night and be blamed for being late to one of them, when all he had to do was admit he didn't get a telephone message. Or call when he did get the message and say he was going to be late. Or if the host had called and made sure he had gotten the message. Or the host called when he didn't arrive and ask what had delayed him.
Replacement Scrappy: Warren Ferguson. The guy didn't have an ounce of what made Barney so funny. The fact that he tried to fill Barney's role, but ultimately failed, is what made his character particularly unfunny.
Lee Van Cleef, in a very small role as—surprise—a bad guy in the carnival episode.
A then-unknown Jack Nicholson in some brief scenes in one of the later episodes.
Pre-M*A*S*H Jamie Farr and William Christopher had made memorable guest appearances; Farr as part of a band of gypsies, and Christopher as Mayberry's new town doctor, Doc Peterson.
Much earlier, a pre-Jeannie Barbara Eden arrives in Mayberry as the town's pretty new manicurist.
Seasonal Rot: A pretty serious case. After Don Knotts dropped out of the show after the fifth season to pursue a film career, the series went on without him for three more years. Most fans dislike these episodes (easily recognized because they were shot in color) for being unfunny and moralistic. Without Barney to play off of, Andy loses much of what made his own character funny, and he becomes a rather solemn grump.
Also, the sudden addition of new neighbors in Mayberry, such as Howard Sprague and Emmett Clark, really didn't help matters much either.
Wretched Hive: Raleigh. Bit-part bad guys hail from there more often than not, and it is often used as the foil to Mayberry's wholesomeness. Nonetheless, the show is as beloved in Raleigh as it is anywhere else in North Carolina.