Adaptation Displacement: That's right, there was a book. There was a book called Hundred and One Dalmatians (without the "one"). People only familiar with the story from the movie might be shocked to find out that Perdita was called 'Missis' (while there was a Perdita in the novel, she was a different character), Cruella had a husband and a pet cat, and a subplot contained Perdita's lost love who went missing. The book had a sequel too, but both the animated and live action sequels were original stories.
Broken Base: Disney buffs can't decide if this film is the start of the Dark Age for the studio. On the one hand, it marked a huge downgrade from the animation of the previous film Sleeping Beauty thanks to the studio using xerography to photocopy the cells rather than hand-inking them to save money. On the other hand it was still one of the highest grossing Disney films, so some regard it as the final great Disney film before the Dark Age (although some give that to The Jungle Book (1967) - the last completed film Walt worked on).
Hollywood Homely: When Pongo is scouting out a mate for Roger (and himself), the first woman to walk by is a gangly brunette in glasses and a beret, carrying an easel and walking an Afghan hound. Pongo's quick to dismiss the pair, but they weren't at all bad looking, and the woman probably wouldn't have been the worst match for Roger, especially considering they're both artists.
Love to Hate: Cruella—aside from the Dalmatian puppies, Cruella's easily the most popular/iconic from the movie.
"Blast this pen, blast this wretched, wretched pen!"
In Israel, kids often call Cruella debil (a word borrowed from French débil via Russian meaning dumbass).
Misaimed Fandom: Many fur fans actually like Cruella, especially the animated version, for her huge fox coat. And a lot of the mentions of fur, like sleeping between ermine sheets in the book, often just seem like a sybaritic Pretty in Mink.
Moral Event Horizon: Cruella, Jasper and Horace are already established as cruel for their part in stealing the puppies and even if they didn't know that they were as sapient as them, the fact still remains that they are willing to murder dogs because of some petty desires. And then they try to run the truck off the road and kill the human driver just to catch the puppies (and if it wasn't for Horace's clumsiness they would have succeeded) sealing the fact that they have no respect for life in general, be it human or animal.
One-Scene Wonder: The four cows only appear in the film for a few minutes to feed the puppies, but they're iconic characters and arguably feature the film's Signature Scene.
Signature Scene: The scene where Perdita leads the puppies where to get milk from the four cows is used in a lot of the film's promotional material.
They Wasted a Perfectly Good Character: With such fun side characters like Tibbs, Colonel, Nanny, Horace and Jasper - the puppies themselves are pretty underdeveloped. They're pretty much Living MacGuffin characters with only a couple of traits to distinguish them. Notably the spin-off cartoon would flesh them out a lot more.
Values Dissonance: While Perdita's giving birth to her and Pongo's original fifteen puppies, Anita and Nanny are helping her while Pongo and Roger are waiting in another room, behind a closed door—it should be noted that during the time period the movie was made and came out in, it was typically frowned upon for men to present when their wives/female partners were in labor with their babies, and were basically flat-out banned from delivery rooms. However, in the modern-age western world, if a biological mother and biological father are still romantically involved at the time of birth of their child, it can be considered the norm that the father is present at the birth (though some may get squeamish, leading to the trope of them passing out during the birth of their child)
Viewer Gender Confusion: Pepper gets one speaking line during the entire film and her voice led many people to believe she was a boy.
Visual Effects of Awesome: All of the cars! Looks like CGI, right? Actually, they're wooden models with hard black lines painted on their edges, photographed moving in real time with every frame then blown up and xeroxed onto animation cels. Despite all this, they haven't aged a day!