Create Topic Tag List Page
Create a new page that lists all topics tags. This will allow a user to discover topic groups which have not yet risen to the level of an official ~group.
Add a new button on the home page sidebar "Browse the list of topic tags". Mockup
On click of the new sidebar button the user is taken to a new page /tags which is very similar to https://tildes.net/groups. The new /tags page lists all tags used in relation to existing topics. Mockup
On the new /tags page the user can click on a tag name which would go to https://tildes.net/?tag=tag-name
Below each tag name are each of the ~groups which the tag has been used in. When the user clicks on one of these, they are taken to ex: /~news?tag=usa
On the right column of the /tags page the count of all related posts could be shown, and also maybe a Filter/Remove Filter button.
The /tags page will require pagination, and if search is still a long way off, a simple text input to filter the tags list would be nice as well. Some sorting options might be useful: Topic count, alphabetical - topic count (descending) might be a good default.