This may sound odd, but there was too much action! One of my favorite aspects about this series has been the slow pace that's still interesting. Yelena learns about poison and magic and develops her abilities. With fire, she doesn't actually study or conquer any magical abilities associated with it. I don't feel like getting into details, because it's complicated, but the entire book was spent traveling, with constant decisions.
Although, to be fair, I'm glad the ending was okay--I really didn't think everything would work out and it'd be another Pirates of the Caribbean 3, where Elizabeth only sees Will once every 10 YEARS and she's still somehow content with that?!? That movie upset me; I'm glad this book's ending didn't. I hate bad endings.