If they can't pin the book on an audience, a publisher might not be so keen on investing.

But that's exactly what I meant in the first place...
To re-phrase: if a publisher thinks they can sell the book, which equals 'thinks that people will want to buy it', then that, to them, equals a well-written book. Good writing to them is writing that people want to buy.

Okay, I get your point. I still see them as different disciplines though. What I meant by well written, is the technical skill of being able to get thoughts, actions, settings etc across in good english. You can be a master of English Language, but a terrible writer of fiction. Or you can be a master of a the English Language, write brilliant fiction, but misjudge what people actually read simply by being out of touch with your genre. There are lots of good books that would never get published today without serious editing. I can think of several that just by title would get knocked back. And not just literature: sit-coms from the seventies, news reports from the sixties, circus side-show acts from Victorian times. Like it or not, the understanding social trends is its own little skill, and separate from writer's ability to write.

Yeah, agree with that Colin.
I have recently read certain books that by no means meet your definition of 'well-written' - barely literate some of them - and yet they have fans and sell loads. I have the feeling that 30 years ago they wouldn't have been published either. Literacy is in trouble.
A certain bookselling chain regularly has spelling mistakes and 'their' for 'they're' kind of mistakes in the messages it sends out to its staff from central office. Also, when there was all that kerfuffle about Meg Rosoff slating bookselling chain staff for their lack of knowledge of books and writing, it was pretty depressing to see certain booksellers including mistakes of the 'their/ they're' sort in their indignant self-defences. I wanted to post 'Guys, you are NOT helping your case' but I was just too depressed to do so (and also didn't want the hate mail! )
People seem to want the story without the writing, nowadays.

Haven't been able to read all this (blimey, what a lot gets posted on WW when you're out of touch for a week) so, going back to the original question, here are some thoughts in no particular order, which may repeat what's been said.

Start wherever appeals to you - whatever vision has just come. Write all the scenes from all over the book whenever you want to. This is easy because it cashes in on your enthusiasm, but a) you need to have a pretty clear idea of what the elements of the bok will be, and b) even if you have, you can land you in knitting-up problems later, and you may end up having to bin some really good writing because there's no true home for it in the book. Can you trust yourself to recognise those darlings that need murdering?

Do that as a way in, to find a voice, nail a character, whatever, then go back and start at Page One. That first little bit may or may not turn out to belong in the novel. You may want to do that again - take time out from the steady progress onwards from p.1, to write odd scenes as they occur to you. Could be great, could make it very difficult to get back to that steady progress.

Start at (what you think will be) Page One, and keep writing, not stopping or going back or fiddling, till The End of the first draft. FWIW, this is what I do, because a lot of my practice is aimed at reproducing in my writing the first draft, as nearly as possible, the reader's experience in reading it. I do a fair bit of planning first, but all in literal and metaphorical pencil, with a total openness to rubbing out and re-planning as and when it needs.

Whatever you do - and as I can tell from only skimming the thread, WWers have come out in their usual strength and enormous variety on this - recognise that this first novel is your first pancake. The chances are it'll end up in a half-burnt-half-raw blob on the plate. That's because the pan needs seasoning, and you need to learn your trade. (We all do - I blogged about this recently here - http://emmadarwin.typepad.com/thisitchofwriting/2008/07/just-for-the-sake-of-it.html - and the pancake analogy's just got another blog thought going, so many thanks)

Very good luck with it!

Emma