I could say that the characterization in this book was rather shallow; we never get offered any background about Jack, his family, why did he end up marrying a woman if he was gay or how he found out he was gay...
Quite few parts of the plot also depended heavily on the deus-ex-machina invention (call me X) to pull the heroes out of their holes, for it to be a really good suspense.
But the truth is that all things told, I enjoyed reading it and while it was a bit sweeter than the kind of book I usually like, I didn't mind it here at all, and really rooted for Jack and D to have a life together.
Perhaps that's why I hated the ending so much. I have many times argued that a certain book needed an epilogue; this I believe is the first time I've ever said that the book would be so much better without the epilogue. In fact, my advice to new readers, is to avoid reading it completely. You're not missing anything trust me, and you're left with a much better and happier feeling about Jack and D, that you will if you read it.