I really only had two complaints, which is saying something for me. 1) I felt that the language -- the flow of words, if you will -- could have used a bit more polishing. I understand that it's a children's book, but there were passages (and even pages) that just felt a little rough around the edges. 2) For being the villain of the piece, the Witch of the Waste is barely in the book until very near the end. Even her secondary presence through the different curses didn't feel as ominous as it should have. To boot, while everyone else was gloriously complex, the Witch herself seemed rather one-dimensional.
Okay, seriously, those are my only two complaints. This is a fabulous book and is quite a bit different from other children's fantasies out there. Even as an adult, I thoroughly enjoyed it.