This is what agent Karen Grencik says agents/editors are looking for in picture books:
“Very child-centered stories where the kids will see themselves in the place of the main character, with a very strong narrative arc, with scenes specifically building one upon the other, a series of three, and a very emotionally satisfying conclusion, with broad audience appeal and a great marketing hook. They do not want fleshed out descriptive texts. They want simply the framework of the story so that the illustrations can offer 50% of the story. They want tension throughout and great page turns. All under 600 words, preferably. Oh, and a takeaway for the kid. They want layers to the story where a child or parent will see something new with each reread. Sweet stories or fun wordplay stories are not enough anymore.”
My story Hurricane Carlos, recently rejected by Karen, has a lot of these features, but it may be that I fleshed it out too much. On to revision!