One thing that I've noticed with StackOverflow is organization is a big priority of the site, however there is little incentive to retag items. There is a badge for the first retagging, however there is no incentive to continue.
My suggestion: This is very similar to the image tagging problem in computer vision. (Applying appropriate and correct tags to large collections of images). This was solved by using the ESP Game and comparing the results of the tags with others' reactions. Google uses this to tag images.
I think this would be pretty cool, and would solve the issue with organization and tags.