It's like two different UIs mixed together. The pet window and spell window look like they both came from some other UI - to me, they really stick out and kind of marr the otherwise very nice look. I don't like the spellbook much, but that's because I prefer the small icons, but I really do like the pet window - I just feel it should be in a different UI. Perhaps the icons should too, as their cartoon-ish feel doesn't match the UI, but I can't say I don't love them, so they can stay
Love the fact that you've managed to put the inventory up for use at any time without making it take over most of the screen, good job.