One key feature of Stack Overflow tags (and I do have a "taxonomist" badge on a few Stack Exchange sites :)) is that is it bottom-up, but supervised: - only po

One key feature of Stack Overflow tags (and I do have a "taxonomist" badge on a few Stack Exchange sites :)) is that is it bottom-up, but supervised: - only power-enough users can create new tags (synonyms are mapped, useless tags are pruned), - questions are mercilessly re-tagged they don't follow tagging standards, - there is requirement of at least 1 tag, and a limit of max 5 tags. Exactly because of that I was able to create a graph from tag coincidences: http://p.migdal.pl/tagoverflow/ http://p.migdal.pl/tagoverflow/. I tried to do a similar thing for many other systems (with wild tags, e.g. Twitter hashtags) and results were never satisfactory.