This has to be one of the worst maxims perpetuated by writing teachers and other writers ever. It ranks just above "kill your darlings" for me, but only because I understand (and agree with - sometimes) the idea of deleting characters or lines that may resonate with the writer but not with the writing. Why we still utilize these phrases as a kind of regimented approach to writing is bothersome. But perhaps that's the nature of the writer learning in an academic environment. While I don't think there is ever one and only one "true" way to write, I understand the need to at least condense the basic ideas of writing into an easily accessible patois that every new writer should pick up and hold on to for awhile before discarding completely.
"Write what you know" was a terrible barrier I had to scale in order to find my own voice within my writing. For years I tried to tell stories that contained elements of my own personal knowledge...and they were awful. I was simply saying the same things I'd read about in other books; same themes, same basic happenings, same feeling of "blah, who cares?" When I reached grad school, I became steadfast in writing the stories that I wanted to write, regardless of whether they fit my particular "style." More often than not, the pieces were met with resistance because they were so wildly strange or because it was obvious that I wasn't rooting my stories in any real kind of situation. Pure imagination with, perhaps, a bit of subconscious finding its way in, but never overtly.
My undergrad thesis was built upon a sci-fi aesthetic with heavy moral and ethical undertones. My philosophy and political science backgrounds helped, obviously, but the story itself was my main concern: was I telling the story I wanted to tell? Almost, but not entirely. Was it good? No, but looking back on the manuscript recently showed me that I was heading in the right direction. At almost 300 pages long, it still tugs at me every once in awhile, whispering for me to come back and clean up its mess, to revise its lines and give it a conclusion worthy of the heavy issues I wove into its storyline. But very little of it was based in actual empirical thought or experience. I had to do research on many separate subjects in order to round out the story as a whole, otherwise nothing could be believed and the story would be anemic in detail.
Once I set upon my Master's thesis, my mentality had completely flipped. My stories weren't based on any particular background other than the base desire to write a story that I myself wanted to read. I found this infinitely more valuable than any maxim taught in a writing class, and for the moment, I still prefer this. The separation of my real life from my writing life has been the best thing for me personally. Obviously, this wouldn't work if someone considers themselves to be a non-fiction writer in the vein of David Sedaris or Augusten Burroughs, but for a fiction writer, removing "write what you know" from your literary vocabulary and your entire mind-set can be unbelievably liberating.
What this particular aesthetic does is quantitative. It forces me to do research, which is something I've come to enjoy a great deal. Not only have I learned more about the things I previously only had a passing interest in, but it fills out my writing, makes it feel full and vibrant. There is the possibility of being too pedantic and focusing too much on the unnecessary details, but once you've done the research, all those ideas sit and stew up inside your subconscious to be used later. The subconscious is a fascinating bit of our brains and it's constantly moving, constantly solving problems our conscious brain can't seem to solve on its own. Your brain is a computer that doesn't have a "delete" button. Everything that goes in stays in and never gets put in a trash-bin or accidentally filed in the wrong place. Your subconscious knows this and helps to make sense of all the information on your behalf. Utilize it. Fill your free time with a small amount of research on even the smallest scene in your book and you might find yourself surprised at how many personal writing questions get answered while you sleep or you're not actively thinking about your book. It's astounding.
Another benefit to writing what you want is the qualitative aspect - I'm writing the story about which I am the most passionate. If I begin writing a story that I have only a passing interest in, the writing will show my lack of interest. While there are parts of my life that some might find incredibly engaging, I personally find them to be better in an anecdotal or oral sense and would never put them to paper. If you can read your own work and think "Something resonates here," then you're probably in good shape. But if you read your own work and you can see how badly the language is being treated on the page, you know it's time to scrap a good chunk of it and start over. Being passionate about a project and loving a project are two very different animals. At the beginning of 2012, I was passionate about this four-section horror novel set around a single house in different eras. I liked the idea, but the writing simply wasn't there. It all rang false in my head when I re-read everything. Likewise, Surya read it the same way and I scrapped all but one of the sections, which later got fleshed out and published.
As I've mentioned before in previous posts, I try to shy away from writing about my life so as to not offend or illuminate the lives of the people I know without their consent. People who don't write typically don't understand the nature of dissecting or cherry-picking personality quirks to be used in sometimes wildly different characters and this can also be a serious pratfall if one sticks to the "write what you know" maxim. There's certainly nothing wrong with writing about people you know - the woman you see at the dog park, the old man at the coffeehouse, the child walking home from school every day - but I've found it best to stay away from those that you know intimately. You have an imagination, you should use it. There is also a world of information out there in which you've doubtless barely scratched the surface.
Choose a section of your writing that seems to be lacking something. Pick a tiny part of that lack, research it for an afternoon, a weekend. Sleep on the issues you seem to be having, let your brain do some heavy lifting while you rest. In one sense, you'll end up actually writing what you know as you will utilize the information you researched previously...but this isn't typically what they mean when they say "write what you know." Don't be afraid to start something you know nothing about. Be a student of the world and find a way to enjoy the research that necessarily comes with bucking this phrase and tossing it to the wind. You may find yourself infinitely more agreeable to the process when you're not stuck trying to write about ideas you already know something about.