I've been thinking about this a lot more. And even researched it. The concept itself is called "Folksonomy"
http://en.wikipedia.org/wiki/Folksonomy , derived from social networking sites. But, the concept can be applied to any kind of bit of information.
There are two major advantages I could see this bringing to a forum:
1) No need to worry about what board topics belong in. Why? Well, consider the site to be an array of topics not categorized yet into forums/boards. If each topic was tagged, and there were tags in common, the topics can be grouped together based on relevant subject. A search of a tag keyword will return results of all topics which have that tag. If you think about it, the forum structure now could be simulated by this method if each topic's one-and-only-tag were the forum it's in. However, what can't be done currently, is the same thread existing in 2 different forums, since tagging is all about being able to tag a topic with as many tags as you want. This allows you to make a new thread without browsing into any forums, then as you make the thread just pick a few relevant tags and post - the website will figure out where it goes from there. Think about it. If I confused you, lemme know :)
2) Customizable display of pages. Does the "recent posts" list not work well for you because you're only interested in otherkin topics and not the chit-chat? Or maybe you're new here and just want to read up on certain subjects for knowledge? Maybe you wanna see if something's been talked about before a lot easier? Tags can easily do this. It's a very searchable mechanism. Search engines basically return a list of results that contain the words you search for. Tags are just words too, but there's a lot more words than there's usually tags. If there's a fewer number of tags, then it becomes a lot easier to search and the results are much more accurate and organized. You can have multiple "blocks" on the site containing posts that match a customized search function.