What's needed here is a reference point. Something that sites can use as a framework of topics. Something they can publish as cross-reference points from their own categories. Or, ideally, their category usage would already BE in this shared vocabulary (like WordNet for example).

This is easy enough, just emit the items with the URI for the topic applied to them. Of course you'll need to get around the stupidity of idiots like Dave Winer that falsely believe all this should be simple. Hey, I want simplicty as much as the next guy, but not if it means being so stupid.

The next step is some form of auditing that prevent idiots from improperly polluting the topic space. Without it someone could just ballot-stuff the topic space by using all sorts of topics on items that don't even come close to falling within the space. Think of how web sites try endlessly to ruin search engine indexes by stuff tons of keywords onto pages that have nothing to do with them.

Here we run into the need for some sort of reputation system. One that seeks to differentiate the valuable material from those just trying to incorrectly grab attention. I'm all for new people, outside of A-lister clique behaviors, to join topic spaces. I've seen all too well what happens when a newcomer wants to participate in a category only to have some asshole significant figure try to force them out. Reputation assignment is not a trivial task to undertake and I'm not suggesting I've got it anywhere near thought out. I recognize it's needed but applying it will be an adventure.

Then there's the temporal, or time, related aspect of this. A given person's ability to participate in a category/topic space is certainly going to be affected by the passage of time. A person posting something related to a given topic deserves some notice. A person known to be informed about a given topic, but not currently posting on it, likewise deserves notice. Then there's the person that posts on something but is widely regarded as not knowing their ass from a hole in the ground (in that topic). Yeah, it gets messy pretty quick.

I think I may know of a way to do this but it'd depend on people waking up to the value of using more sematic web-like principles. I've no idea how to get them to wake up out of the cult-like brainwashing they're stuck in presently. Some folks are starting to get it but until much wider audiences buy into the ideas this is just going to take much too long.