One of the most eye-opening things that I learned on my path to publication was 32-page picture book layout--making a dummy even though I'm not an illustrator. It really helped me realize which scenes were too long or too short and when my page turns fizzled instead of surprised. I have a page about it on my blog and it's the most frequently accessed page: http://taralazar.wordpress.com/2009/02/22/picture-book-construction-know-your-layout/
This is almost common knowledge now, I realize, but at that point in 2009 I was like...DUH!!!!
I also like to "think about the hook BEFORE you write the book." As a result, many of my stories tend to be high-concept.
And PB writers should know their story should have an emotional core, a universal emotion that kids can relate to. There's the topic--which is what the book is about on its surface (like dinosaurs, ballerinas, trucks, princesses) and then the theme--what it's about at its core (friendship, family, fitting in, perseverance, etc.).
I LOVE PICTURE BOOKS. HAVE FUN!