I've been meaning to write about this topic for awhile now. A new post on Nancy Kress's blog has finally roused me from my dogmatic slumber to do so.
Write what you know. That was the first thing that was drummed into our little heads in creative writing classes in high school and university. In other words, no great flights of fancy; stick to experiences you yourself have had, or which you can extrapolate immediately from those experiences. That's what our teachers and professors meant by it.
The corollary to such admonishment, of course, was that writing Fantasy or Science Fiction was not approved of. It was assumed to be the opposite of 'writing what you know'.
What resulted from this advice, always, was a form of not-very-creative creative writing; fiction thinly separated from memoir. It always bored me, and eventually I gave up on the whole program.
If I want to read memoir, please, let me read the real thing: an actual history of someone's life. I don't see the point, particularly, in reading a fictional memoir of a false life. It's a pretty lie trying too hard to seem the truth.
In one sense, writing what you know is good advice: one can go very wrong in making stuff up. If one wants to write about being an auto mechanic, well, one ought to know something about the auto mechanic's life, or one's story will either be thin on detail, or wrong on the facts. And neither of those is any use to anyone.
But much of the great literature of the past was written by people who wrote about things they did not -- or could not -- know about. The great tradition of mythic literature would have been impossible otherwise. Homer didn't know anything about the Cyclops, Circe or the Sirens, nor the gods of the Iliad. He had heard stories, no doubt, but as for personal experiences, he made them all up.
And that, after all, is the point of anything fictional: it's all made up. At its most basic, fiction is a series of lies. For the lies to be compelling, they must be told convincingly, and that alone is the point of writing what you know. Write convincingly. But write with broad scope.