The 4-star rating is for the book's Mistborn connections, rather than the book as a stand alone. There are plenty of references to past characters and the marks they left on this new Victorian society, and there's an unexpected appearance of an "old friend" ol' Iron Eyes... So Sazed brought you back, did he). The book as a standalone gets a strong 3.5-stars because it's only the tip of the iceberg, and there's still so much Sanderson could tap into, like for instance the central mystery is left open-ended. The missing women are still missing. And then there's the introduction of electricity. It would be interesting to see how Allomancers function in a wired world. Wouldn't they get shocked more often when burning metals? Along that line, we never hear about Allomancers getting hit by lightning, even when they burn metals outdoors in stormy weather. Vin never mentioned lightning, only rain... Anyway, tangent.
Wax is a sheriff/bounty hunter living rough out in the Roughs, a lawless dessert in the style of the Old West. A bounty hunting expedition goes terribly wrong that leaves him with post-traumatic stress and forces him to hang up his guns, possibly for good but not likely, and return to the city Elendel, named after the Lord Mistborn of course. Wax goes back to the city a few months later and, because of his uncle's death, takes his place as head of a renown house on the edge of bankruptcy. And so he must marry a young lady from a lesser known house of great wealth. This young lady is not Marasi from the back cover description, but her cousin. Thus a triangle begins? We don't actually get to that point, and at this point, we don't care...?
While Wax is coping with post-traumatic stress and sorting out his house problems, there is a curious series of train robberies that's possibly tied to an even more curious kidnapping of noble women in the city. Wax tries to stay out of the way and let the police handle it, but actually, no one in fiction has ever done that before. The police are somewhat incompetent and slowed down by protocols, whereas a lawman like Wax relies on instincts and sense of duty (and allomancy) to get the job done. Wax's snappy smartass of a sidekick, Wayne (it's adorable, OK), pushes him on to solve the curious case, but it isn't until a robbery and kidnapping hit close to home that pulls Wax into action.
Does he solve the case in the end? Not really. He ties up a piece of the puzzle. This is why this book can't possibly be a stand alone. Why introduce so many important elements only to end with part of the mystery solved and then introduce another main player connected to the mystery.
Allomancy and Feruchemy are further explained and examined through fight scenes. What's even more intense is that various Allomancy + Feruchemy combinations are shown. I am in awe of the depth of Sanderson's mind.
Miles is an effective foil. I hesitate to call him a villain because of his beliefs in bringing change to the world. At one point, he even compares himself to Kelsier and the similarities make sense. Society is corrupt, there's an obvious class distinction, people in power are negligent, and so it's up to him to restart it, just like Kelsier. But is he a righteous (or self-righteous) hero of Kelsier's caliber? Maybe. Both believed they fight for just causes.
As for Marasi, if this book is to become a series, there's still time for her to grow, to become a main player, a world shaker in the style of Vin. She's obviously not anywhere near that point yet, what with her constant blushing and Victorian propriety in check.
Harmony. Sazed's new religion makes sense. It takes his Terris values and belief in simplisticity and creates a religion that puts good deeds on equal footing as tithes. Makes sense. Very Sazed.
[ETA: 08/12] Just had an interesting discussion with a self-proclaimed "huge Sanderson fan," and he said Sanderson never meant for this book to have a sequel because he originally intended to write a novella. To which I said, "HAH. Right..." If Sanderson never intended for this book to be a series within a series, he would have given it a satisfying ending and that would be that--no sequels required. But it doesn't have a satisfying ending, does it. There are one too many sub-plots left hanging off a cliff.