How Low Can You Go?

To make a good story, we have to build tension.  The villain must be truly evil, and we have to find ways to empathize with the hero.  The way we do that, obviously, is through the sharing of what's happening.  However, I wonder just how much we have to delve in to the situation, and how much we leave to imagination.

(Empathy is the key)

Just what can authors get away with when building tension?  We can kill the hero's wife, but what about his 9 month old child?  And if we can kill the baby, can we describe the gruesome details?  Would the audience allow us to talk about flesh bubbling as the kid was put in the microwave?  Would they want the villain dead, or would they want the author dead?

There's a revulsion factor when it comes to reading some things.  Alluding to a child-murdering rapist could help us hate him, but describing one of his crimes in detail would probably be crossing the line.  In other words, you have to build true tension and character empathy without crossing the line and make the reader focus more on you as the writer than the characters you describe.  When Salvation Day comes out, I want people to focus on the storyline, not whether what was written about Hell is verboten.

This is a subjective line, and our target audience will decide what's too far.  If they read something and don't like it, we'll be lucky to sell another book.  We want them enthralled, not disgusted.

What lines do you think exist?  Have you ever encountered them?