The city of Ondinium, capital of Yeovil, is a strictly socially stratified city. The exalteds, the cardinals and the plebians rarely mix, and the lictors are always around to "prevent indiscriminate mixing of castes". As an icarus, Taya is one of the few citizens who has the freedom to cross caste lines on a regular basis -- the icarii deliver messages to everyone.
The mystery itself wasn't a tough one to figure out. I had the baddie identified from the first introduction of the character. That didn't detract from my enjoyment, though (which is odd, as that's usually a huge pet peeve of mine), because I had so much fun trying to puzzle the clues given and my gut instinct into a more detailed theory about Why and How and so on.
The other odd reaction I had to Clockwork Heart was that even though I never got particularly attached to or invested in the characters, that didn't detract from my enjoyment either. Usually a lack of connection with the characters is a huge problem for me -- here, not so much.
I think I was able to get past both of those issues so easily because of the world building, which was, hands down, my favorite aspect of the book. Dru Pagliassotti dropped me right into (well, technically above -- Taya is flying when the book begins) Ondinium and let me get acquainted with the world as I read. As the book progressed, the world got bigger and bigger, the technology became more and more interesting, the politics got increasingly more complex, and my perspective was constantly affected by snippets about the history of the world and by the different opinions of the characters. The interactions between different castes had me fascinated and I'm very curious about the world outside Ondinium.
I really hope there will be more books set in this world, whether they are a continuation of Taya's story or about completely different characters.