There are a few false notes, I think -- Sarah's (good) psychiatrist would have been fine in a film version but I think needed a bit more personalization to avoid being a stock character on paper, and Sarah's stepmother... She's used as the wicked stepmother throughout the story, and it's been long enough since I've seen the film that I can't call how in character that is, but it's supported well enough in the story for me to buy it, given the motivation the author gave her. Since that was done and done well enough for me to believe it, though, it makes her caving at the end, telling Sarah that she'd always wanted what was best for her and just couldn't bring herself to believe in the fantasy, even when it was proved to be real... a bit too easy. I don't think this story needs that kind of ending, where all of the loose ends are tied up (except for Jareth just being at the start of figuring out who he is, and that loose end is left nicely open for either a sequel, or just the reader's imagination as to what he's going to do now).
These are fairly small things, though, in a long, plotty story that read like some of the better popular fantasy novels I've enjoyed over the years. Good stuff.