All of the characters are extremely complex and thought out. They all have complicated (in a good way) backgrounds that leave you wanting to know more. And although the situation is a little less than real, unless you've seen the Dead walking around, the characters feel so tangible and real. You actively cheer them on, you cry with them, you feel their frustrations, and you maybe even hate them.
The plot is also riveting and exciting. While there are some slower parts, that's only to be expected, and it gives you a slight break from all the movement. Even if you're not a fan of historical fiction or of zombie books or whatever, this book is so much more than just that. It's also about relationships and society and how we're led to make decisions we would never want to.
Another aspect that I loved was that I found the book to be less descriptive and more character and plot driven. I'm the type of reader that hates to be bogged down by description unless it's really important. Susan Dennard does a fantastic job of describing the important things so that you get a sense of the setting without it being overwhelming. I found that many parts also had more telling than showing. Perhaps you don't view this as a good thing (although there's plenty of showing as well, don't worry), but as someone that struggles with the concept of "show not tell," I really enjoyed the bluntness of some of the statements and facts. It didn't take away from the story, and it didn't leave you hung over on what it means so that it would slow you down.