What Could Have Been: Before the book was set to be about wild monsters, Sendak wanted the land to be full of wild horses, but decided that horses would be too difficult to draw. Seeing as the wild monsters have become synonymous with the book, this is probably for the better.
Write Who You Know: The titular wild things were mainly based on Sendak's aunts and uncles. Also, the dog that Max chases with a fork is based on Sendak's sheepdog, Jenny, who also appeared in several other books, notably Higglety-Pigglety-Pop!.
Specific to the film
Acclaimed Flop: Those who weren't put off with how different the tone was from the book were praising it to high heaven, but it only just barely made back what it cost to make, which—due to marketing costs and giving movie theaters their cut—meant it lost a lot of money.
Box Office Bomb: Only made $100.1 million against a production budget of $100 million. Being released at the same week as Avatar definitely didn't help.
Dawson Casting: Max Records was thirteen years old when he played the much younger Max.
Descended Creator: Spike Jonze provides the squawks of Bob and Terry, the two owls.
KNB EFX was initially offered the chance to do the creature effects based on their work on The Chronicles of Narnia films. The company turned it down as they felt adapting the book was a bad idea.
All of the failed film adaptations that came before the Spike Jonze version:
Several small time indie film makers apparently pitched scripts to Sendak, which all got shot down.
John Lasseter of Pixar attempted to adapt the film back in the 80's for Disney, but Disney shot it down due to the difficulty expanding the book.
Universal got pretty far into production on an all CGI animated film that got canceled, turned into a live action version, and canceled again.
Arcade Fire was allegedly slated to do the soundtrack. They did re-record their classic song "Wake Up" from their debut album Funeral... which has only been available as a 30 minute sample publicly, resulting in a strange case of Missing Episode for that version of the track.
