I think being able to up/down vote tags would be a nice idea and similar things have been suggested when discussing genres.

Up-voting would just add the tag to your own tags. This would make it easier to add tags that are already there because you wouldn't need to type out the tag name on every page you want to use it. It would also hopefully encourage more people to tag - right now it's not clear that we do still want people to add tags even if they already exist.

Down-voting would help with things like MBS-4144 where editors get annoyed with bad tags but can't really do anything about them. If the overall average of a tag gets too low, we could hide it from everyone except the person who added it. They wouldn't show up in the sidebar and there could be a link on the tags tab saying something like "2 tags not shown" which would show them if someone really wants to see them.