Google Docs is for writing documents, letters, school essays, etc... The output is a document of some kind.
Sheets is for performing calculations, modeling numbers, etc... the output is normally some calculated value (or values). The benefit of this over just writing a single function/program is that you can potentially have more insight into the way that calculation is performed, but in general, the output is some calculation.
But outliners are different... what is the result? The NLS demo describes a shopping list. Which is nice, and you can slice and dice the data around, but fundamentally, it's a shopping list. And that function can be achieved with Google Docs for 80% of the functionality. Sure, it doesn't make moving bananas and carrots into a 'produce' category easy, but with copy and paste, it's a pretty well solved problem.
But the basic outliner functionality you're describing -- what's it actually used for? What's the payload that you're organizing?
Even Dave Winer's tools were used not for making lists, but for making websites. There was a "payload" present. You were managing data for websites (at least, as far as I can understand it, I didn't have a Mac at the time, so never used most of the UserLand programs).
So, would you say the "point" for these tools is making other tools? For example, using Frontier to manage websites? Or Wave to make a chatting tool?
In my mind, this was the problem that Wave came across... it wasn't easy to describe what it did in a simple way. It wasn't email. It wasn't chat. It wasn't collaborative writing. It was the backend that made that all work. It was incredibly ambitious, and that made it difficult to get people to use.
If it was "just" chatting -- I think that would have made a huge difference. Users don't care that you're using OT to write messages on the fly... they just want to be able to chat with friends or colleagues.
So, what is the killer application for a collaborative outliner? Why would I need one? I'm genuinely curious as to your thoughts, as you've clearly spent a lot of time thinking about this.