Tags for topics

Would it be a nice feature to be able to add tags to topics? I think this would be a beneficial addition to the forum. I imagine it something like this:
Any logged in user can when reading a topic add a tag to the topic which will make the topic more probable to show up when searching for that tag in the forums.
If the tag was already added sobe earlier than the new addition is simply ignored.
Maybe have an upper limit on amount of tags for each topic.
What do people think about this idea? Would it be hard to implement?