I don't use WP.com much now, except for as an archive/testing site, because I prefer having my own site on my own domain. However, it is interesting to see what things they've implemented within the site for managing inter-blog interactions.
Tag Surfer: allows you to subscribe to a tag, sitewide. WP.com's implementation of tags I find irritating, because when you click on a tag, you instead get the global tag stream instead of posts on the blog tagged with that tag. However, the idea of a global tag stream is one w/ great potential.
Blog Surfer: This allows you to subscribe to WP.com blogs, which appear as an aggregated feed, including private blogs which do not have RSS feeds. So vaguely like an LJ-style friends list, except sans external feeds. I'm not sure, though, how far it can go. There's an option for "posts since last login," though.
Readomattic: Confusingly, this seems to be partially an alternate way of accessing Blog Surfer (or Friend Surfer: they seem to be confused about the terminology). However, you can also view, or subscribe to, categories of top Wordpress posts, tags (confusingly overlapping with Tag Surfer), and twitter. Twitter? I don't really get why I would want to subscribe to Twitter via WP.com, but now I can.
Except for the ability to view private blogs, I'm not sure how Readomattic/BlogSurfer is superior to simply using my own RSS reader, except that it's faster. At least if I wanted to view something like the "top business posts" vertical, though, they allow me to block posts from blogs I don't like. I wonder if private blogging on WP.com really is that popular. Since the blogs are so invisible, I have no way of knowing. ( rambling about Wordpress.com, Wordpress, privacy features, and LJ )