I think I understand why this book has received so many mixed reviews. 
 First of all, let's make things clear. 
 1. The book is cataloged as being YA... but it's not! I honestly don't think it falls on that category just because the main character is 19. In fact, Kelsea is a bit older than most of the protagonists of YA books. The story also depicts violence, stories of rape and pedophilia, profanities, and a lot of other things not usually found in stories for teenagers. Kelsea is more mature and less "innocent" than most of YA protagonists and has no clear love interest. She does have a superficial crush, like any young woman, but it's not the main driver in the story and it isn't explored much. The only thing YA about this book (the only thing I didn't like) was Kelsea's whining about her plain appearance, but I think she'll grow out of it. She has already shown that her insecurities won't stop her when things need to be done. 
 2. The book is merchandised as being high fantasy. Don't get me wrong, there is fantasy and it it's a sort of medievalish world... but the story is, in fact, set in the future after an event referred to "The Crossing". At first it was weird to read about "genes" and things that a medieval world shouldn't know... until you understand what's going on later in the book. 
 3. It is slow. There is a lot of plotting, secrets, machinations, and politics. I loved it, but I think the pace is not for everyone. 
 So, imagine being promised a high fantasy, YA book and ending up with a science fiction, fantasy, adult, violent book with no romance and lots of politics... I understand some people being disappointed. I loved it. 
 Lately I have not been very fond of books with lots of action and little time for characters to develop or show themselves fully to the reader. This story fit my mood perfectly, but be warned: it may not be the same for you. Basically, the story follows Kelsea as she is escorted to take her place as queen of the Tearling and finding out the state of her kingdom (which has been kept secret from her). She grows from her childish veneration of her mother, the late queen, to uncover the truth (or at least part of it) about her reign. 
 The book is an introduction to the story, setting the foundation of the world, its history, Kelsea's ideals as queen and all the secrets that she needs to uncover. 
 Oh, and by the way... I'm in love with the Mace. You can have the Fetch, just leave the Mace with me.