What an excellent world building! That's the first thing that comes to my mind after reading this book so I'll mainly talk about that. 
 We find ourselves in a land where everything is brown and unfertile, ashes fall from a clouded sky, and a mysterious mist haunts the night. People are enslaved by the nobility, who themselves are controlled by obligators and inquisitors, everyone ruled by an immortal and godlike ruler. There are people called "mistings" and "mistborns" with special powers drained from metals, mainly in the nobility. Inbreeding between nobles and skaa (the slaves), is strictly controlled/forbidden in order for the ability not to be held by skaa. The world is really complex, I could talk and write hours about it but I try to keep reviews short. Let's just say that it's a depressing, sad and dead world where people are oppressed by a really powerful man (or god?). 
 We follow a crew of thieves as they plan a joint venture with the rebellion in order to take over the city and get their hands on the stock of the most priced and powerful metal. Atium. One of the crew members is a Mistborn in training, so we get to learn the use of allomancy (ability to draw power from metals) through her eyes. As the story goes on we learn more about the world's history and about how the Lord Ruler came to be. We also learn about the Deepness, an undefined threat that was fought long ago just before the ascension of the Lord Ruler. 
 It's true that the middle of the book was a bit slow. It focuses on politics, plans and how the threads between characters are moved from behind the scenes. It shows the reader the history of how the world came to be, the world itself as it is now, and how allomancy works. Basically, I felt that this book serves as an introduction to the world and the actual story. The ending looks like the opening of a gate to the real deal and leaves a lot of loose ends that seem will play a big role in the next books of the series. I think we need to read the next books to understand the full depth of Sanderson's world building, so if you'll excuse me, I'm going to start the next one. 
 PD: there is a little bit of romance, but almost nonexistent. I would have like at least a kiss, but well, I'm a romantic.