Dan Hon's article Inflection Point asks: What if we didn't have to save documents? What if all versions were always kept?
Firstly, the barriers between documents disappear. There's no point starting a new document if you can just edit one that is mostly what you want and email that off. So files come to represent activities, each a tree of connected documents that are variously printed, emailed, considered.
This is the meat of Dan's article -- how would the UI change in this case? I'm an fan of the division of metadata into pay-for and free varieties. Word frequencies, date of creation, these are metadata that occur as a by-product of the task at hand (writing). On the other hand, file name and location have to be added, if you want to find your document again. If these could be added for free, if documents were so easy to discover even without explicitly naming them -- what other pieces of metadata could be demanded from the operator to make their life easier down the road? And this in itself creates UI change: if the similarity of documents is valuable, then it has to be easier to mutate a document for a new version that to create one afresh.
Which is the real point. Given this endlessly mutable document, how to tell when there's a useful revision and not an in-progress one? Example. When I use my paper notepad at work, if I'm four or five pages on from a set of notes I want to add to, I'll still start a new page, and rewrite what I need to from my old notes. Putting the lid on a document, archiving it, this serves some kind of purpose. And when it comes to computers which don't by necessity have to follow any metaphor at all, it's important to break the symmetry and place these purposeful things in... As ugly and unaesthetic different documents, opening and saving are, maybe not having infinite undo (in this paradigm) serves some kind of conceptual purpose?
But really this is about making computers more like the human aspects [putting a lid on something, for example, but not emulating loss] of the RL environment. Instead of binary save/open there's a less digital approach. Or to put it another way, clots in the Lifestreams; points of inflection on the activity curve where the document creator has paused to print, email, consider, consult, spellcheck. What Dan's talking about is softening the computer interface to take the best psychological parts of the utter mess that is my work desk, perversely good filing system that it is, and add it to what we know computers are good at. So hopefully not continuing the query versus hierarchy dichotomy, but taking a synthesis of it. There's an angle here, I think, an interesting one for thinking about UI.