What to say in the end? I've raised different facets that play into the discussion of how much violence in Christian fiction and is there a line to cross? Honestly there have been several good comments that hit on where I was planning on going with this.
1. I still believe in the idea of the Christian artist having the freedom to write what they feel like they need to in order to make the story what it is supposed to be. That doesn't mean freedom from critical reviewing, because any time someone puts out a creative effort it is open to critique.
2. It depends on the genre and audience the author is writing for. I wouldn't expect graphic violence in a prairie romance, (unless written by Chris Mikesell ;-) ), and it would probably turn off the intended readers. If I pick up a Robert Liparulo novel, I am definitely anticipating it, and that is my choice as a reader and consumer if I make that choice.
Some genres like war or suspense pretty much require some violence, mortal danger, etc. It wouldn't be true to the story if it wasn't included. If it makes sense with the overall story, then it probably is needed. Yet a necessary scene of violence can become gratuitous if overblown. That may well be a point of taste that is impossible to quantitatively determine, but falls under the old adage, "I'll know it when I see it."
3. I am reminded of the quote from Jurassic Park regarding cloning: "Just because we can doesn't mean we should." I will state continuously that Christian artists should creative in freedom and integrity. Just because we can have greater freedom in publishing to write violent scenes doesn't necessarily mean it should be done. I like what Merrie said in a comment, about the toolbox of a writer and using different techniques at the right time. Using all sorts of dramatic settings or plot twists can enhance the story. She mentioned Hitchcock showing just enough to scare without being voyeuristic about things. Sometimes the subtlety is a better way of portraying a scene than hitting the reader with a gross-out hammer.
Sure, tastes have changed since his day, and audiences are supposedly more "sophisticated". Maybe our culture is just dulled from being able to appreciate subtlety and the build-up of suspense over showing the violence.
4. Just tonight I was watching a show on the life and martyrdom of William Tyndale. Here was a godly man who lived in constant fear and danger of being discovered, yet still managing to translate the Bible into English and write books that would change history. Finally he was betrayed by a friend, imprisoned for 500 days, and when brought for execution had the privilege to be garroted so that he was dead when he was burned at the stake. I'm reminded again how our faith is not the sanitized, dressed up in "Sunday go meeting" clothes faith we live in America. Our predecessors suffered terribly for our rights and abilities to serve Jesus, and there are millions today who also are persecuted to the point of death for His name. To ignore this dramatic history and its legacy, to whitewash the blood of the martyrs, it would be a horrible injustice to the strength of our witness.
5. Overall this was a question without any definitive answer. People in the comments hit that right away. I don't have answers that will satisfy. We won't have labels or ratings on books. Reviews may or may not expose issues for sensitive readers. I'll defend an Christian artist's freedom to do something even if I think it may have crossed a line, and I may bring it up as a reviewer. Some books won't be for everyone.
Ultimately as Christians it comes down to us being sensitive to the Holy Spirit and His guidance. If we write something that is integral to a great story, yet we realize that it will grieve the Spirit, do we serve the muse or the Lord? (Reminds me of the ending to Stranger Than Fiction) As I try to write, I want to increase my skill and what I can portray with words, but it has to come down to how it works out in my relationship with God. If being true to that means writing books that leave the squeamish behind, so be it. If it means sacrificing a little artistry to being a disciple, then make it so.