This is an excellent idea. If anyone uses not just plain tags and categories, but rather needs sets of semantic entities (such as persons, locations, companies, products) that may even have been extracted from blog postings automatically, this is the way to go.
However, in my opinion this is not yet a 1.0 version as some essential components are missing:
- display widgets for entities (lists/trees or tag clouds)
- merged display widgets with categories and tags
- migration tool for the conversion of tags into such entities and possibly back
- integration into search
Keep going! :-)