To a new writer?
So many things!
Remember that publishing is a business, not a charity. All the publishers are interested in is the bottom line - how much money they can make, directly or indirectly, off your masterpiece. They aren't in it to be kind, they aren't in it to spot the next Shakespeare.
Learn your market. So many new writers write something wonderful but unclassifiable, not even cross genre. Such a shame, because they fall between the cracks. Or anyway, if you don't learn your market, create one and claim it as the next big thing! But learn where you fit in the market, read it, know what's happening in it, who the movers and shakers are.
Be prepared to adapt. If you want to write a wonderful book about the Battle of Waterloo and a top agent asks you if you want to expand the romance you featured a little bit, don't dismiss her suggestion out of hand. Think about it and if it would fit, do it.