Point of clarification. When talking about "just different enough", it might be easy to think I'm stumping for the kind of far-out creativity that is epitomized by everything from Naked Lunch to bizarro fiction. Well, not really.
For one, let me take Naked Lunch and William Burroughs as its own case, separate from the hordes of things it inspired. The more I look at that book and that author, the more I see them as lightning in a bottle — a unique set of circumstances that most people have learned the wrong lessons from. The lesson of Naked Lunch wasn't that you needed to get really messed up on drugs or have freaky experiences to write well; it was that a book like that was only possible because of the circumstances of the author's life — and those circumstances weren't things that you could, or should, duplicate. The point is not to live freaky and die freaky; the point is to live well and observe thoroughly.
The other things about Burroughs that everyone seems to miss is that the man was just as hidebound by his approach as the most by-rote romance novelist. One book like Naked Lunch is one thing, but the problem was that with very few exceptions, he kept trying to recapture the same lightning in a bottle over and over, with diminishing returns each time. A few of his later works are intriguing -- Cities of the Red Night, for instance — but even those books are still trapped inside the mind-set Burroughs brought to everything, a mind-set which only looks expansive at first but turns out to be reductive and limited in its own way.
That's why the whole bizarro movement seemed to me like wasted breath, and why freaky for freaky's sake feels like no less a dead end than copying everyone else's playbook. Writing weird for weird's sake doesn't guarantee anything interesting will happen on the page, because the real reason a book is interesting is because of the author behind it. An interesting person writes an interesting book, no matter how straight or freaky it is on the surface. A boring person — an incurious person, a person hung up on his own view of things, no matter how outlandish it seems on the surface — cannot help but write a boring book.
Real diversity isn't about appearances, but attitudes — about making the best possible case for your vision by making it as broad as possible. That's why I find SF&F all too often to be disappointing in that measure: instead of being about widening the scope of our vision, it ends up being about narrowing it to recapitulate everything we've seen in previous years' books.