I'm playing with Radio Userland and having fun with the outliner. I'm slowly picking up what all the fuss was about. It's not that outlining is revolutionary or fun, it's just that sometimes it's easier to structure your thoughts with a little help from the editor.
Word does outlines, but it's like using a bulldozer to brush lint off your shirt. Emacs does outlines, but like everything else in emacs it requires fourteen hands and a fetish for the Ctrl key to use. I'd use vi, only I don't use vi because I was abused by a beep when I was a child and the trauma still haunts me.
OPML? Haven't played much with it. Heard that (much like Radio's HTML) it's a bit of a hit-and-miss thing in terms of standards compliance. I just like thinking in terms of nested lists. Don't hate me because I'm hierarchical.