MrsHighwater, I am genuinely glad that you liked it. Don't let my killjoy rantings take anything away from your enjoyment!
Plot devices, though, since you asked...(long answer alert)...Well, I like books in which the central character comes alive in my imagination. Great literature is full of great characters, some of whom are so strong and so vivid that you can imagine them outside the confines of the novel they are in. Elizabeth Bennet from Pride and Prejudice is a great example. There was a series on ITV last year caled 'Lost in Austen' whose premise was a modern-day London chick swapping lives with Elizabeth Bennet. The reason that TV series worked was that P&P has such amazing, mutlidimensional characters that it is possible to extrapolate how they might reach to a completely different context.
Part of the joy of reading great books is that you can anticipate how their chrarcters are going to respond to the situations they are in. You get under their skin, you weep and laugh with them, you love them or hate them.
Personally, I didn't find that was the case with 'The Shack'. I felt that what the author really wanted to do was to get a message across and he chose the genre of fiction to do it in, rather than having a great story to tell with a message which, for those who have ears to hear, deepens and enlivens the story. I felt that the characters were one-dimensional. It's a bt like watching Casualty on the telly, and seeing a cetain nurse walk onto the set, and being able to predict who the next person onto the set will be, because the storylines are that crass and obvious that people only relate to each other in set ways and can only exist with in the framework of one storyline at a time.
THe effect of all this on me is that I really can't be bothered with what happens to Mr XYZ because in my imagination I've already sussed out that he's not a real person, he's a cut-out figure whose only purpose in this book is to introduce all these theological discourses. So I gave up reading because I wasn't sufficienly concerned about the fate of the daighter because, let's face it, the book wasn't really about that, it was a framework for a message.
Really good Christian literature - I'm thinking about the parables of Jesus, Dosteovsky, C. S. Lewis and one or two others - is all about genuinely good stories that entertain and satisfy, but have a deeper meaning lurking under the surface. Adrian Plass said something about "stories that entertain at the front door while the truth skips in through the window". The Shack was a bit more like the truth barging its way through the front door with the story wandering in behind it looking slightly puzzled.
So the quick answer to your question; here you go!