This is the first of several updates coming soon to make the topic-tagging system more functional, since it's been only informational up until this point.
You can now filter a topic listing down to showing only posts with a particular tag by clicking on that tag in the listing. It will affect the current listing you're on, whether that's your home page or inside a group. So for example, clicking a "facebook" tag on your home page will take you to this page - a list of all topics with the tag "facebook" in your subscriptions. Doing it from inside ~tech would go to this page instead, which is "facebook"-tagged topics inside only ~tech.
One thing to note is that (as mentioned in the mechanics page), tags can be hierarchical and this filtering supports that. For example, filtering to "rock" in ~music will also show a post I made yesterday tagged "rock.progressive".
Let me know if you notice any oddities with it or have any feedback about how it works.