I think this book did well upping the stakes in terms of the history and mythology. It was nice to have it set in Idris, which you read so much about throughout the first two books. The introduction of Sebastian was intriguing, if a bit forced--I sometimes felt that Clare needed a scapegoat and he felt too deus ex machina for Jace and Clary's are-they-aren't-they-sibling relationship.
Other than that, I thought the action and pacing was good. I was definitely hooked. I could've done with a 100 or so more pages. I've been finding that most books, after resolving the problem don't really delve into the consequences after the climax. However, there are three more planned books set in the same timeline, so I will just wait those to come out.