A SubjectSpace is just a list of subjects. Each subject should have a self-contained description. As opposed to a description that depends on the context defined by its position in a SubjectHierarchy. The problem with a SubjectHierarchy, is that different individuals' hierarchies are very unlikely to match up, but individual subjects have a better chance of matching, and users can always rename their subjects at some later date. In effect each user's SubjectSpace is a push-style channel that other users can subscribe to. Unless they want to.
WikiWikiWeb in effect has a shared SubjectSpace.
See MiskiProject.