Best Known for the Fanservice: If the movie gets mentioned, most of the time it's going to be for Clark Gable's shocking Shirtless Scene - where he wasn't wearing an undershirt. Legend has it that it caused undershirts to drop in popularity.
There's also the famous, and often parodied, scene of Claudette Colbert flagging down a car by lifting up her skirt and exposing her bare leg.
Ear Worm: The bus singalong of "The Daring Young Man on the Flying Trapeze."
Peter's Epic Fail at hitchhiking. By the end he gives up and gives a rude gesture to the last couple cars.
Seinfeld Is Unfunny: This codified (if not invented) most of the tropes associated with romantic comedies, which doesn't occur to people who complain how cliched it is.
Values Dissonance: Peter opines to Ellie's father that she should perhaps be "slapped around a little", which doesn't affect her father's view of Peter at all (probably because it's entirely unclear whether he's serious, being hyperbolic in his frustration, or metaphorically saying that she needs more structure in her life).