What do book editors want?

What do editors really want? Why are they putting one thing in the rejection pile and preparing for an auction for another? The difficulty in answering these questions is that every editor opens every proposal the same way you open presents from relatives: with hope but trepidation. They never know when it’s going to be great, but great is exactly what they’re looking for.
Whenever I come back from a lunch with a book editor, my agency colleagues always ask, “What are they looking for over there these days?