Too bad they didn't implement this 8 months ago, I might not have switched to Wordpress for the recording of anything I consider worth reading. I suppose the question is what I do now...
If it were possible to only have people read a specific tag (including RSS feeds of it) in their aggregators... Right now it basically only serves as a hackish categorization system for reading the latest entries on a user's journal, but doesn't (as far as I can tell) export RSS or allow you to only friend those specific tags, that might be worthwhile. (PlanetRDF, for example, only sees my SemWeb posts, where PlanetMobile only sees my mobile posts.) Hopefully in some future permutation, we'll see this.
(I wrote this last night, when I thought tags were actually going to go live. So much for doing things on time. I guess that's LiveJournal time for you...)