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What Forum Software should I use?

I wonder if anything like this exists. I'd like to add a forum to my site, but instead of the usual forum/subforum/sub-subforum structure, I'd like to use a metadata/tag approach where everything exists as a single directory, and where there's a search field at the top which instantly (<0.5 sec) filters the threads to a particular keyword or keywords.

Also, as the admin, I would be able to add highly visible buttons at the top, which can be clicked on for the main categories I choose for the forum (nevertheless, users can also add tags to their own threads outside of these default main tags I supply if they wish).

This approach, if done properly, is more powerful, efficient, maintenance free, scalable and friendly than a standard forum, so I was hoping someone had the same idea and made something out of it. It couldn't be that hard.

I'd want the speed to be up to (or near) the standard of this: http://forum.dlang.org/

Other forums (e.g.: phpBB) are orders of magnitude worse than that in terms of latency (posting or browsing), and I think that is wrong, even in principle ;)