There are two mistakes I see a lot of writers--often students--making, and when you get down to it they're the same mistake.
The first is the tendency of the writer who has inside information to hew so closely to that information that they forget the rest of the world. This information could be anything, but these days it often involves culture and class--the increasing diversity of our society has created a great demand for culturally specific material, and such material, in the possession of a skilled writer, gives their work a special authority. Think of Louise Erdrich, for instance, or Jhumpa Lahiri. These are good writers who have used their inside information to breathe life into universally interesting stories.
But sometimes a writer gives the inside information itself too much weight, and it turns into exploitation. Look! it can seem to say. Hyphenated Americans! Alternative Lifestyles! The inside information no longer offers readers a unique point of access to the human condition--instead, it feels like a barrier through which the reader cannot pass.
The other mistake is perhaps a reaction to writers like Lahiri and Erdrich. It's the assumption, among those who don't have any inside information, that their own experience is worthless, and they have to go fake it in somebody else's territory. The result is every bit as sterile as the exploitive-insider story, and perhaps more boring.
The worst thing about this mistake is that the writer is wrong--there IS value in their own experience. They just haven't hunted it down yet. Their inside information is as inside as it gets--it's their own way of seeing. Indeed, that's the true inside information for everyone, no matter what else they know. The trouble is that you don't always know how you see things. Every writer needs to figure this out: you have to trick yourself into not tricking yourself.
Lately I have hugely enjoyed the work of writers who have worked their way through their cultural otherness and emerged as, definitively, themselves. Zadie Smith and Colson Whitehead fit the bill, I think--they are black writers whose work addresses race, but is about, and for, humanity. And Frederick Seidel, who I've blogged about a couple of times, is a rich white guy who, against all odds, renders fascinating a kind of life that our literate culture is otherwise liable to dismiss. He does it--as the other writers I've mentioned do--by being absolutely, inalienably, himself.