Perigee Bookmark, in the past week, has released two lists that I think every writer who wants to get published (especially in the traditional way) needs to read. Perigee is an imprint of Penguin, one of the Big Six, so their editors tend to know what they're talking about.
The first list is Why We Say No, which details the Top 10 reasons why an editor passes on a project. The usuals are there: no platform, bad writing, or no sales potential. A book on on how to slaughter puppies probably isn't going to sell well, for example.
But, the list does offer some new ones: my favorites being that the editor or publisher already has a similar book or there's simply too much competition in that particular field. Writers don't really think about those ideas too often - we're told to submit to places that publish books similar to ours. In reality, a publisher isn't going to want books that are in competition with each other.
And, if the market faces an overwhelming amount of one type of book, then the publisher might decide that they're not going to join in. This is another point authors need to remember, especially when they try to emulate what's on the bestseller lists. Remember that by the time your book sees print, so many books just like that bestseller will already be on the market.
The second list is much more uplifting: Why We Say Yes. It also includes a lot of basic answers, but the editor admits that sometimes, it simply comes down to loving that book. If you can write a book that grabs the editor's attention to the point that they simply have to publish it, trust me - they will publish it. They'll might even fight other publishers for it (yay, bidding war!).
So that's really what it comes down to: write the best book you can. That's what publishers want to see, and what will get your book published by the traditional presses.