The main character Tessa is essentially an empty vessel into which the reader can step; she lacks any real personality of her own. She has little agency, merely reacting to events around her and never acting on her own. Her only trait is that she's determined to find her brother, and the author does not sufficiently establish an emotional connection between them for the reader to care. In fact, there's no connection between the reader and Tessa, either. What was Tessa like before she came to England? What were her dreams, what were her fears? The author takes great pains to give us the impression that Tessa reads books, but how does that impact Tessa as a person? How does it influence how she behaves or what she says? None of that is incorporated into Tessa's behavioral make-up; instead, Clare only uses this to allow Tessa to make comparisons between the novels she reads and what's happening around her. The constant novel title mentions is the literary equivalent of celebrity name-dropping - really, Clare, you're trying too hard to impress your audience.
After ragdoll Tessa comes Will, the cheeky and witty love interest who is more often obnoxious than charming. His dialogue is tediously clever, as though every line is either a joke or its punchline. While characters like that can be a fun addition to a novel, Will is not developed deeply enough to make him anything other than a shallow pretty boy. The romance between him and Tessa develops out of nowhere and does not feel authentic.
Jessamine is perhaps the only one that had a spark of personality in her; she was the only one I was interested in learning more about. Quite frankly, I think her story [a girl determined to fulfill the typical Victorian role for a woman who is repeatedly thrust into this weird and dangerous other world) is the one that should have been told.
Too many characters all together in the book. Clare doesn't handle multi-character scenes well. Too much is going on and the writing becomes confusing.
And the information dumps! Pages and pages of people sitting around (or standing around) explaining things. I get that Clare wants the reader to understand the world she painstakingly created, but really. That's too much.