Unique difficulties of writing a novel

Coat of arms modelled in waxComments on last week's post got me thinking about what writing a novel is like. I said it was similar to my own area of expertise, wax modelling, where I rough out the shape by joining strips of wax, add and subtract until it looks right, then texture.
But on reflection writing is more difficult than that. If you are modelling a coat of arms, like the one on the left, you can tell at a glance when you have got there, because it looks as it is supposed to, and anyone can see that. You know when you are on track, as the piece begins to resemble the working drawing. 
With a book, when you are finished all you have is a lot of words. You've been toiling on it sentence by sentence, chapter by chapter, while the first reader experiences it whole. Send it out on submission to agents, and the odds are none of them will think you've got it right. To be an author, not only do you need superb judgment of what to put in and what to leave out, but at some level you have to have a crazy amount of faith in yourself.