One of my students last year wrote her final paper on why Christians should exercise discernment in their media involvement. We had a lot of loooooong conversations about discernment, and she really had to work hard to address a common objection about fiction in general -- the "it's just a story, it's not real" objection.
Just taking Twilight as a case study... one of my issues with Twilight is that young girls don't need someone telling them that that's how love is supposed to be -- that there's a guy out there who's perfect in every way, who's your soulmate without whom your life is utterly meaningless, and that its ok if that guy wants to hurt you as long as he has self control. And that when a guy ignores you and barely speaks to you except with apparent hatred, it means that he's just seething with lust. And that it's ok to string a decent guy along until you decide that you do want to be with your perfect sparkly soulmate after all. Teenage girls already are so prone to thinking all that. It's already programmed into their little texting, MTV (do kids even watch MTV anymore?), Jersey Shore, Bieber-obsessed worldview. They don't need an adult to confirm it, they need lots of adults to correct it!
I have some pretty big issues with the weirdo Mormon theology that's EVERYWHERE in the books, but the relationship stuff is my major practical concern. I don't really have much of an issue with adults reading them, since they're more experienced and discerning, and can tell the difference between fantasy and reality. But girls who are 13, 14, 15, when they're just left to read the books on their own with no one talking to them about the issues it brings up? Not so much.
Anyone who knows me knows that I'm not the type to go, "ZOMG it haz teh witchez/magic/fantasy BURNNNNN IT!" I mean, I did read all the Harry Potter books. ;)
But I do think as Christians we have a responsibility to ask questions. And the question isn't, "Does this movie/book/whatever depict a world without evil or darkness or moral complexity -- a nice, shiny, clean, Precious Moments world where everything turns out just dandy?"
We need to ask questions like, "Does it portray evil as evil and good as good, or does it pretty up evil or trick us into thinking something evil is really not so bad? Does it show the reality of the battle between good and evil? Is it realistic that sometimes evil seems to triumph? Does it show human character honestly -- that we're all messed up by sin and make mistakes, even the heroes of the story?" Again, that's worldview stuff we're talking about here. "How does the author view life? humanity? love? sex? relationships? purpose?"
For example: I think American Beauty is a absolutely brilliant movie, and that Christians ought to watch it (if their consciences permit, of course). It's rated R, it depicts adultery, drug use, deception, violence and lots of other truly evil stuff. But it also shows, vividly and poignantly, the meaninglessness of a life apart from Christ.
Stories are powerful. We're shaped by them and they impact us in a way that just straight teaching might not. So we have to be discerning, even about fiction -- maybe even especially about fiction, because it can affect us without our even being aware of it. I think about how I feel after I watch a movie -- even the fluffiest, silliest, most blatantly unrealistic romantic comedy can change my mood. It can make me feel dissatisfied with my life, frustrated that I'm still single, annoyed that some sexy leading man hasn't come and swept me off my feet (well... yet... ;p). Stuff like that affects our hearts.
I mentioned American Beauty. It's really good, but what keeps it from being "capital-G Good," is that it offers a counterfeit solution to the problem it presents. It says, "Life is ultimately meaningless. If you can find meaning in the meaninglessness, you're one of the lucky ones." We know as Christians that that's not true, that true meaning and purpose and hope are available, and found in Christ. But just like American Beauty, every story -- from comic strips to epic novels to TV shows -- offers some kind of "answer" to the life's problems.
Only Christians can offer the real solution, the Grand Story into which all of our little stories can be fitted by the Great Author of the universe. But a book, a movie, a TV show, whatever -- all these things are only good inasmuch as they can point their readers toward God's truth. Stories have the power to prime human hearts to see the emptiness of life apart from Christ, like American Beauty, or the reality of the battle between good and evil, like Harry Potter, or the brokenness of a fallen world and our often-futile attempts to fix it, like Sherlock Holmes, or the inherent beauty and preciousness of human life, like Children of Men.
Ultimately, we have the freedom in Christ to read, watch, or listen to just about anything. And we have a responsibility to use that freedom wisely. So, read Twilight or watch Mad Men or listen to Katy Perry or whatever. But do it with your eyes and ears wide open, and do it like a Christian.