Heya! Let's have a look how it feels to post feature requests in this forum :)
Currently tags can be added to a forum discussion when the thread is started. The problem is that we usually forget to add some relevant tags. And moreover, it easily happens that a thread gets an unexpected shift in focus.
For these situations it would be great to add tags to a discussion, after it is created! Maybe with each new post, you can also add some tags.
What is more important is that there are forum moderators who take care of the tagging: Merge some tags, remove others that don't make sense at all etc. Of course forum moderators need the tools to do this.