I believe that if you fully understand this, it will do two things for you:
- help you when you are rejected, especially after those rejections which are accompanied by a message along the lines of "I liked many aspects of this but in the end I feel I have to pass"
- encourage you not to submit your work until you are as sure as you possibly can be that it is as compelling and perfect as you can make it
And if you're thinking maybe she's fussier than other agents, think again. All good agents and publishers make equal demands of the books they agree to take on. There's so much uncertainty in the world of reading that if there isn't passion and certainty in the agent and editor's hearts, then how can they possibly throw themselves behind a particular book? And if they don't throw themselves behind it, it'll be doomed.
Some other blog posts that help explain reasons why your book my be rejected:
I've now had lots of submissions for my Submissions Spotlights, and I can tell you with 100% certainty that none of them is good enough to be published - yet! - though a few contain potential. But you can't expect agent / editor to see through the imperfections to the potential: they need more than that because they simply can't spend the time training you up. All the training and all the practice have to come from you. You can't afford to send anything less than the best.
Hot? It needs to be boiling point before you send it anywhere. Anything less is failure.