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.
This is more of an "it just bugs me," but does anyone else notice how many episodes revolve around Hollywood or someone from Mayberry getting famous? There was James Best's appearances where he joins a band, the time when the movie company wants to film in Mayberry, the time when a music producer wants to get people from Mayberry to invest in their own singing (and they succeed), the time when Aunt Bee wins a bunch of new appliances on a game show, and the half a season or so that Andy, Bea, and Opie go to Hollywood because they want to make a movie about Andy.
Maybe it was because the show was actually filmed in California? It's easier to film exotic locales when you're pretty much already there...
The locations are understandable, but there are so many plots that don't require an exotic location, but revolve around fame and fortune coming to Mayberry. Your response definitely applies to the season where Andy, Bea, and Opie go to Hollywood and spend several episodes there and the same could be said about Bea being on a game show, but it's not really applicable to the other episodes that deal with fame and fortune. Many of those take place in Mayberry.