This book was really a remarkable improvement from City of Bones. The writing got better and it finally lost the "Harry Potter fic" atmosphere that impregnated the previous book quite a bit (yes, I'm aware of Cassandra's background). The plot seemed stronger and more original, and the characters deeper.
I'm starting to like Alec. He's genuinely developing as a character, and I love his relationship with Magnus. OTP! I hope we see more of it.
I'm still not the biggest fan of Clary, but I liked how she acknowledged her issues with prettier females her age and why she didn't have female friends. I'm looking forward to seeing some change of attitude in her since this realization.
I liked Maia and how her story was managed. She grew up in an abusive environment and she ended up in an abusive relationship because she didn't know better. When she understood that and tried to get out, she got turned into a werewolf and had to abandon her family and her life. But she still held on and became a strong and amazing woman. Yes, she still has issues, but who doesn't? And she's just a teenager.
Simon's conversion came along with substantial character development, and I was pleased with his decision in the epilogue.
I was totally moved when they said the Herondales had been in charge of the London Institute. [SPOILERS FROM CLOCKWORK PRINCESS] OMG, Will and Tessa's descendants!! I just can't!
Clary saving everyone in the end was a good choice. She's starting to be more of an active character, and that's great.