I’ve not yet had much chance to play with it, but my ideas are mostly work related-
1) Controlled Vocabulary – a database of tags based on some authority (ie LCSH, Getty, APT, Geonames etc). More important if sharing tags than if using it as a stand alone.
2) Extraction of Exif data from digital photos to create tags. May need an authority file to reduce the number of tags created this way.
Possibly a few more after I’ve tried it a bit more 🙂