From the ones that I've read, usually no explicit sex and at least one character who's an example of happily living the Christian faith. There can be characters who don't follow any religion or are antagonistic to religion, but sooner or later they come to acknowledge at least that there is a higher power and that the character who is Christian is admirable and might be clued into something that they are not.
I try to not read it because I always get the feeling that the story has been sanitized for my safety, and I keep waiting for the preaching shoe to drop.
@ Bronte: You might like the Ray Quinn mysteries by Mark Mynheir, "The Night Watchman" and "The Corruptible". I reviewed them for Amazon and I liked the stories and the characters. Not preachy at all; I didn't even realize the first book was supposed to be Christian fiction when I chose it to review. I know that Ray Quinn is going to see the light sooner or later, but the author is realistically portraying his resistance to doing so.