Adult Fear: Remember, Cruella didn't steal the majority of those puppies; she purchased them from legitimate breeders who probably assumed she was just a rich, eccentric woman who loved dogs. The idea of unwittingly handing over a puppy to a person who might abuse it—much less to a person who intends to kill it and skin it—is the ultimate nightmare scenario for many animal lovers.
Moral Event Horizon: In-story, Missus claims that biting a human is always this for a dog. She is perfectly willing to bite Cruella or the Badduns, but she justifies that on the grounds that they aren't really human (and in Cruella's case, at least, she may be right, as the book and a deleted song take the liberty to imply that her last name might just be meant literally....)
Tribute to Fido: Dodie Smith's inspiration for the book was her own dalmatian Pongo; naturally, she put him into it as the main character.
The Woobie: Perdita. She is forcibly separated from her husband; then her puppies disappear, and she runs around looking for them until she collapses in a storm and is nearly run over by passing cars. After she is taken in by the Dearlys as a nurse dog, her milk fails, making her panic that she'll be thrown away. Then she gets the unenviable task of trying to comfort the family when Pongo and Missis leave to find their children. (But everything turns out okay in the end.)