Some improvement ideas
Hi
When you zoom in / out, the board often zooms in / out to white space and you can get lost. A Home button to reset you to the top left would help.
Being able to drag the underlying page by holding (e.g.) RHS mouse button to move around would be useful.
Labelled vertical and horizontal dividers would be useful to help separate releases and functional areas.
A title and detailed description on the cards would be useful.
Long term, some form of integration with products like Scrumwise for turning the user story map into a backlog. Unless of course you are planning to add scrum anyway :)
There is a bug whereby sometimes when you enter text and move on to another card, the text vanishes and only sometimes comes back when you re-click on the card (FF 26.0)
A help feature accessible from the board is essential!!!!
Otherwise, an excellent tool! Been waiting for this for ages.
Steve