While we are looking at Tags based on Erik P. Ernst idea on moderation, I would like to consider the concept of subscription to tags. Tagging is becoming more commonplace and could work similarly to the recently added @mention feature for community members. This would be the ability out of the box for users to have an interface and logic to subscribe to tags. The concept is any user could subscribe to a tag that would generate notifications (in system and emails) that would then tell them about a trending topic (i.e. tag).