As many commenters have mentioned (as does the article) hierarchical tags are a pain, if not an impossibility to get right. Related tags, though, can be done on the cheap and are surprisingly powerful, fun and cool under the right conditions.
Say you have a massive database of photos, each photo having tags. As example we'll use the tag "United States", which is used as a tag on 50,000 photos. Next, you go over each of those 50,000 photos and check which other tags were used, and sort them by occurrence.
This reveals useful and often surprising implicit relations between tags. The relation can be of any type, hierarchical or otherwise. It reveals relations never explicitly mapped or maintained. It's organic, which kind of fits the philosophy of tagging.
Say you have a massive database of photos, each photo having tags. As example we'll use the tag "United States", which is used as a tag on 50,000 photos. Next, you go over each of those 50,000 photos and check which other tags were used, and sort them by occurrence.
This reveals useful and often surprising implicit relations between tags. The relation can be of any type, hierarchical or otherwise. It reveals relations never explicitly mapped or maintained. It's organic, which kind of fits the philosophy of tagging.