"Coming from the perspective of a developer, the problem with OPML is that it is focused on outlines and not on distributed lists. There are already two standards for distributed lists; RSS and Atom. The moment you introduce hierarchy in a single document format you lose the distributed nature that most of the use cases rely upon and you open the door for content getting out of sync which introduces a new problem that is very difficult to solve and decreases the usability of such solutions. An RSS enclosure or an Atom link can reference any content type, not just a media file. If the entries of a feed contained links that simply referenced other feeds then developers could leverage existing technologies and paradigms as well as provide a better fit for the common use cases of distributed lists." - scott anderson