Dave Winer and Scott Rosenberg disagree that an outlining metaphor forces you into hierarchical thinking. This discussion is entirely coincident, but relevant, to yesterday’s discussion between Jon Udell and Don Thomas about how the tree metaphor works great for users who think a certain way (which turns out to be a lot of people).
In many cases, structured hierarchies are hard to understand and navigate, like a massive file system or a deeply nested taxonomy. But in the case of lightweight tools like outliners that let you do the nesting and grouping, it is a structure you have created yourself. In my own experience with this metaphor (mTuitive Authoring Environment), I’ve found that it is best to resist nesting greater than three levels deep, unless absolutely necessary. Similar to the “three-tap rule” in Palm’s early days (nothing is more than three taps away from the home screen), I’ve used this as a guideline in working with subject matter experts who use our tool, and it keeps the tree under control.