I can’t tell you what it is because that would be a huge spoiler. But there’s a major one (and it’s one Condie writes about in the acknowledgements).
I still need to fully process the Matched trilogy (of which Reached is the final book). I’m fascinated by the fact, though, that although you could read it and love it without any knowledge of Mormonism at all, it contains within it major resonances with Mormon history, landscape, doctrine and practice.
A Motley Vision 
Now I’m even more upset that I forgot the last book was coming out and I am number 180 on the hold list at the library! You are such a tease. I’m excited to finish the series and have a discussion about it.
Hopefully, the library has ordered 100 copies of it.
I’m #2 in our library system, and the library aide who signed me up is #1. It may be here on Monday!