Something I think is really great is that because this is all xml, it could potentially be exported to epub format—also xml. Or Word. Anyone who's written something of book-length (whether a book or a thesis) knows that outlining is a fantastic way to do it. Imagine doing it all on the World Outline, opening up OPML comments when you're ready for feedback and then shipping the whole thing to a book format.
Students could use it for group research projects.
Educators could use it to provide real-time feedback to research papers and dissertations as they are written.
Newspapers could just put it all together like this.