Mikel has directed my attention to Dave's revival of OPML files as directory structures in Sunday's edition of Scripting News.
As Dave demonstrates, the 'inclusion' feature of OPML (the link and url node attributes) allow for a really cool decentralized directory structure, with different persons managing different levels of what appears to be a unique outline. This has a lot of potential.
If you have activeRenderer installed with your copy of Radio, you can experience the same level of integration within your web browser as you can using Radio's outliner.
Take a look at my OPML directories demo.
I like OPML because it's so simple, as all great standards should be. But OPML is supposed to be an XML format, which means that several minor quirks need to be ironed out in Radio's implementation, at least in my opinion.
For instance, I'd like outlines saved with Radio's outliner to be encoded using UTF-8, a Unicode compliant standard, instead of ISO-8859-1. I'd also like UTF-8 encoded OPML documents to display the proper accented characters in Radio's outliner.
As a majority of people on this planet, I write in several languages besides English, all of which require a different set of accented characters, all of which have improper support in OPML. Proper handling of UTF-8 would solve this problem.