“Shared journals” don't have all of the spiffy maintainer stuff attached to them that communities do. Of course, with tags now you're supposed to start putting your “tech” entries in your normal journal and tagging them as such! I've yet to actually look into it and find out if that would work in practice, though. I'd certainly like to stop using custom security as my category-faking hack.
Having tags shown with an entry, and linked to the page that shows all posts with that tag, is quite useful.
I guess your post begs the question of what else you wanted tags to do that they don't? The major thing I can think of is to filter your friend "subscriptions" (i.e., for each friend to show only posts with particular tags, or hide posts with particular tags) which I imagine is something they can add later based on this infrastructure.
OH. that would be a GREAT idea. i hate when i can't add someone to my friends list until i've gone back through everything and categorized all of my posts into increasingly complicated filtering systems. it would be lovely just to block them from certain tags.
Oh, that's actually the reverse of what I was saying! I was talking about being able to filter my friends' posts by tags — like, I only want norbert's "music" posts to show up, or I want all of darlene's posts except the "politics" and "rant" ones.
Tags have the advantage of being formalized into the protocol, so you can manipulate them with a client. Specifically, you can (should be able to) post an entry with them, instead of having to remember to edit your memories manually. More usable == better chance they'd be used effectively.