I don't mean a responsible writer shouldn't do research or rely on a list of experts to help make sure that she knows what she is talking about, and conveys that knowledge in a manner useful to her reader. Research is a necessary part of good writing. Otherwise you have a reader who knows that vampires don't walk around in daylight wondering why the ones in your book can, and putting the book down when you don't provide a compelling explanation.
But too often, "write what you know" makes us cowards. We know what will sell, so we write to that instead of the book that lives in our secret heart. We know our strengths as writers, so we hide behind those, and never push ourselves to try new things. We know how putting certain truths in our work will make our parents, our friends, our lovers look at us, and so we don't write those, because we know that writing something else will be easier, and cleaner. We know what is safe to write, and we write that.
We write what we know when we want to be perfect, and I don't think perfection is what art is about. I think art is about truth.
I am not a good writer when I write safely. I'm an educated one - an advanced degree in Literature means I have plenty of practice thinking about the pieces that go into a story. I can give you theme and archetype, blood, love, and rhetoric, all wrapped up in a tidy bit of literary allusion. I'm good enough at language and the manipulation of words to turn a pithy - or pretty - phrase. I can paint the surface of the page with sparkle and shine. It's beautiful, but it's false at it's heart. At best, what I make when I do this will be mediocre.
I don't want to be mediocre. I would rather be a spectacular failure than be safe.
I was reading an interview with Amanda Palmer today, and in it (while talking about twitter) she says, "share things that make you uncomfortable." It's that piece of advice that I would replace "write what you know" with. Because I've found that when you acknowledge the existence of that discomfort - poke around in it a bit and think about why you don't want to write that scene, or that character, or that story - what comes out is better art.
When I say this, I don't mean uncomfortable in terms of shock value. I mean it in terms of truth. I mean it in the way that if you think people will judge you, that they will look at you funny, or like you less, or wonder if you're a creep or a pervert or a weirdo or or or, for putting something in your art, then maybe that is exactly the thing your art needs.
I'm not going to lie, and say that's easy or fun, or that there haven't been times I've handed something in and thought "Dear God, I hope my Mom never reads that." And I don't succeed at being honest in my work all the time - possibly the most devastating critique I ever received was a single line: "Dear Kat, you know you have to write [redacted] if you want this to mean anything." It was devastating because I had known. I just wasn't brave enough to put it in that draft. And it was so obviously the missing piece of story.
So don't be satisfied to write what you know. Share what makes you uncomfortable. Write what terrifies you.
Write what's true.