Although the general idea seems sound, I see certain issues with its practical implementation. Specifically, the question of who will be responsible for assigning tags to posts, and how we will ensure their accuracy if the authors themselves are responsible for tagging their content.
In my experience, users tend not to care about or even notice things like tagging conventions and structure; they just want the content to be visible. I think that if we are going to rely on authors to tag their own content, we will end up with inconsistent tagging and poor search results. I also find it difficult to believe that we will be able to enforce the policy consistently.
Yes, I agree with you that it could lead to poor tagging from those forum members who has no knowledge what tagging is all about, how it's been done and factors that determines the use of certain tags/keywords in a particular content. But with time, when they learn how to use it, they will all get use to it.
However, I think a perfect solution to this will be if authors are made to add tags to their content, while moderators modify the tags, by adding the right tags that will make it visible for search.