You mentioned on the blog that agents cry into shots of JD about What Will Not Sell. The answer may be too long to answer, but if it is not, please tell us What Will Not Sell.
I think we all know that warming over something that was not hot to begin with would be on that list, especially if it is the tenth rehash of the twentieth overdo. They do it in the movies all the time and spend tens of millions, but publishers would not consider spending a few thousand on it, eh? What else? Comments would be welcome
There is no One True Answer and if there was I'd be on the writers conference lecture circuit charging a filthy amount of lucre for you to find out.
However, almost every editor has a list of books they've loved that have failed. The longer the editor's been in the biz, the longer the list. The trick is listening carefully as they weep into their shot glass to discern what category those books fall into.
I know bad writing sells.
I know derivative writing sells.
I know stuff that makes me wonder if English is the writer's first language has sold.
The trick is to find work I like and want to read that will also sell. I'm lucky to be working in crime fiction cause I (mostly) like the stuff that sells in this genre. I like it and I can pick it out of the slush pile (mostly.)
I like poetry a lot too. I can pick what I like but is it good, or will it sell...not something I know.
Same with Westerns, romance, women's fiction, or books about sloths.
The real question you're asking though is "How can I not waste my time writing something that you can't sell."
There are two answers:
1. Other agents sell things I can't sell. Barbara Poelle and Sorche Fairbank regularly knock my sox off with amazing books I would have missed.
2. I've sold things other people can't sell.
Write the book you love. Write something that isn't what everyone else is writing. That means you MUST READ YOUR CATEGORY and read it deeply. Know what you love about your category. Know what you don't. Think about your book as part of a larger whole.