I love the story mapping idea, although it’s probably not the best name for it. After all, it is a technique best used when all you have for each requirement is a few words on a sticky note, but no properly-drafted INVEST-spec user stories.
Why do I love it so much? Because it is the best way I know of to define the breadth of your requirements and do some high-level prioritisation before putting those requirements into the accepted user story format. The picture at the head of this post is one I took after a session gathering requirements for a Claims project. It was a very rewarding day and convinced the sceptical business people that we really did have their interests at heart.
Here is Jeff Patton’s definitive article on the subject. I urge you to read it and put it into practice whenever it is suitable.