Given how quickly Ellie falls for Peter, one could interpret her marriage to Westley as her taking the first option she could to escape from her oppressive father - and thus never seeing Westley as anything except an escape route. Then when she meets a man she actually likes, she realises she has no use for Westley.
It's possible Alexander Andrews had no idea his daughter felt so stifled by being raised around bodyguards, making him Innocently Insensitive. He could have attributed her little acts of rebellion as typical teenage stuff, and doesn't suffer a Heel Realization until Ellie literally jumps off a boat, swims to shore and hitch hikes all the way across the country.
Ellie almost overnight goes from Ice Queen to Genki Girl after the Walls of Jericho scene (something Peter lampshades). This is after she's found out he's a reporter. Is she intentionally acting as nice as possible to make sure he doesn't write anything too derogatory about her?
Award Snub: Famously at the time. Bette Davis was thought of as a shoe-in as Best Actress for Of Human Bondage. But due to studio politics and campaigns against her, Claudette Colbert won for this. The snub was influential in causing the Academy to change its voting practises.
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."
Jerkass Woobie: Peter comes across as a dick to modern audiences, but it's heavily implied that he's never known any kind of love before - or never been able to make a relationship work. The Woobie part increases when he hears that Ellie has gone back to her father and husband.
"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.
Unintentionally Sympathetic: It's hard not to feel a little bit sorry for Westley when Ellie runs out on their wedding ceremony literally during the "I dos". It does get mitigated a bit when Alexander pays him a very nice $100,000 for the annulment.
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).
Also, not a lot of of romantic comedies are likely to start from a pretty serious blackmailing these days...
There's also Peter spanking Ellie while carrying her across the river...
Peter and Ellie have to pretend to be husband and wife whenever they stay anywhere, because no respectable place would take in an unmarried couple. Even though the audience knows Peter and Ellie won't have premarital sex, the implication is there.
And notably despite pretending to be a married couple, both rooms they rent have separate beds. The Wall of Jericho gag wouldn't be possible in a film made away from the Hays Code's strict censorship about sexual content.