Less words? More illustrative possibilities? A clear market? A special hook? Character development? What exactly do editors want in a picture book? Melanie Hope Greenburg's article Diamond in the Slush: What Picture Book Editors Are Really Looking For has some great nuggets of information from two editors at top publishing houses.
Just a few:
Can you read the story 10 or more times and not tire of it?
Why is your story different?
Can you identify your audience?
Allow white space for the illustrator to create.
Kid appeal but sophisticated enough for adult readers.
Check out the other tips in the article. Thanks to my critique friend Joanne for sending the link my way :)