The tag communities have really begun to take off on WordPress.com, with people using it to discover other WP.com bloggers who are writing about the same things they are. Since the launch though we've had two items on our TODO that would make global tags extra useful. (WordPress.com: We work weekends so you don't have to ;).)
The first isn't exciting at all, it's just paging. Before you could only see the first page of results for a tag, now you can see them all. Sorry for not launching with this.
The second is actually pretty useful. Every tag page now has a link to a RSS feed where you can subscribe to the latest posts from around WordPress.com on a given tag. It's a full-text feed, just as if you were subscribing to the blog directly. Not only would this be useful to track topics you love in an aggregator like Bloglines, but you could also add it to your sidebar using the RSS widget.
Please continue to provide feedback if there is any way you think tags or feeds could be more useful!