Tags work well for blogs, but I don't think they are a good idea for a message board, especially one the size of this. Between the search function and being able to bookmark threads, it seems tags would not be necessary (just my opinion, of course).
Tags are basically a filing system and, like any filing system, the more people doing the filing and the more people doing the retrieving, the more confusing it gets! Would it maybe be more efficient to refine the capabilities of the search function?