Cimorene

It's not just the ending - it's the pacing and characterization. Hermione surprisingly comes off as real, and there are some lovely lines and a couple of nice ideas and individual scenes, but it feels like it should be properly novel length and a rather wonderful book about coming of age without magic and then facing your worst nightmare when you're not sure who you can count on and -

That's the trouble with flawed but fairly good stories. There's always a brilliant one buried just underneath.