* BaseBreaker: Chachi-nated, or De-Chachi-nated? Nick at Nite even made a marathon out of this premise, and viewers got to vote.
* CrowningMomentOfFunny: Fonzie's been using "Sit on It!" for years, to great reaction. When ''Marion'' of all people uses it on ''him'', the reaction from the crowd and Fonzie himself was priceless.
* DarkerAndEdgier: By the final season. Well, as D&E as you could be with this show. More dramatic moments (among them an AttemptedRape of Joanie), the use of "hell" and "damn", and Fonzie showing more weakness.
* EarWorm: C'mon, sing it with me: "[[TitleThemeTune Sunday, Monday, Happy Days...]]".
** The "Pumps Your Blood" song.
** Fonzie in the first two seasons, so much that he became a BreakoutCharacter and was promoted to second billing, after Ron Howard.
** Mork was only supposed to be a one-shot character and his episode AllJustADream, but Creator/RobinWilliams' performance was such a hit that the episode was revised to make the events real after all and gave him his own spinoff.
* ReplacementScrappy: Roger.
* TheScrappy: Roger.
** Chachi was ''always'' [[TheScrappy The Chachi]]. He was popular with the girls, but retroactively, amongst older fans? Please.
* SeasonalRot: It started out as a very cool, low-key show, then became the ideal version of a standard sitcom with a StudioAudience. It'd started getting worse (the Trope-Named Shark-Jump) even before Richie left, but only got worse after that. Fonzie got a temporary full-time girlfriend for one season, and things got increasingly worse with gimmicks and nonsense until a thankfully fantastic final episode.
* SomeAnvilsNeedToBeDropped: According to Garry Marshall, "Richie Almost Dies" was written for this reason. He explained that he was contacted by counselors working with abused children who watched the show. These kids refused to cry and kept their emotions under wraps, so Marshall wanted to have Fonzie go through an emotional breakdown in order to show these kids that it was okay to cry.