A hosted system where you can write and edit a book-length document and then one-click publish to various platforms, including direct sales, as well as dead-tree prints, would be pretty amazing.
Yup! And that system need not have two hundred features and formats for the author to learn and master. I've been stoked by the idea for quite sometime.
That's kinda what leanpub.com does, isn't it?
Sort of. Leanpub does parts of that flow, but afaik they don't have an editor or draft versions, nor do they actually publish to the Kindle or iBooks store for you.