I think what I'd say is the most "novel userspace paradigm" in Plan 9 is its pervasive synthetic filesystems. You have FTP filesystems and so on with FUSE now, but writing something as flexible (technically) as Rio still requires something other than FUSE. But more importantly, since Plan 9 *started* with those synthetic filesystems they're used everywhere, whereas they're pretty uncommon in Linux etc. It would be nice if web browsers used a kind of webfs, and so on.
Actually, what this discussion keep pointing out is the elegance of the Plan9 authentication model vs. UNIX's superuser scheme. It's the lack of a superuser that makes the whole namespace paradigm work in the first place.
--lyndon
