If you get a bunch of writer's in a room with a publisher, and you start talking about the submission process, the question always seems to come up... why 100,000 words? It's a fairly easy answer really... it's the word count that usually gets us the right page count, at our favourite font, at the right page size to make an economical book. Because book sales are price sensitive, it's important that we get the right print cost, as well as all the other costs associated with publishing the book, into the formula to make the retail work. Retail is usually determined by taking the print cost and multiplying by a factor of 8 or 10. This allows us to then work backwards from the retail subtracting our costs (distribution, which can come in at a whooping 68-70% of the retail price, royalties, production, labour and overhead are just some of what is calculated in) - and hope, pray to the book gods, that we will come out a few pennies ahead. Sometimes we do, other times we don't, but the average needed word count is 100k to meet that goal.
Here's something that throws a kink into the whole darn thing... and right now, since I am in the middle of laying out the first three books for spring I'm noticing it (all of them significantly over 100k hmmmmhmmm people!) that word length is one thing. Two other factors that affect this are the number of chapters, and the number of POV shifts. Too many of either and the page count balloons, and then I'm forced to use an itty bitty font size instead of something bigger and looser. So I'm stuck on these three with 10pt - sorry readers! On the other hand, they are worth a little eye-strain! so do get a copy when they hit the shelves please! (sorry for the shameless plug!)
So, some words of advice.
1. Stick to the publishers submission guidelines, including word count, as much as possible. (I absolutely cannot stress this point enough)
2. Don't have a million chapter starts. Please, we're begging you. Write longer chapters.
3. Try not to use POV shifts. If you have to, minimize. While they can be very strategic and important to the story (particularly complex stories, and I realize that they are sometimes very necessary) it has an impact on the overall cost-effectiveness of publishing the book. And that means it can have an impact on whether or not you get published.
Eventually, it gets to the point where as a publisher you don't really think of these things, it's just such a normal part of day-to-day life, so we may forget that the info is important to you writers. If you've got a question you always wanted answered about how or why we do something, ask away. If I can answer it, I will.