There have been repeated references to the possibility of implementing object/content tagging via one of the proposed new features (e.g. secondary types, or custom objects).

I can imagine some requirements, including:

• hierarchical tags
• facility to relate a tag to an object (i.e. relationship, secondary type or something else)
• query tags (e.g. top N tags by usage)
• control security of tags
• private versus public tags
• specify some standard tags (e.g. 'My Favorites')
• tag any object type (including custom objects if implemented)