People care about things. People tag the things they care about. "Johnny Smith" on his underwear. "Joe Campbell's Lunch Box." "This book belongs to Bob Radcliffe."
People tag the things they own or would really like to own...like things on a wishlist.
Tim at Librarything.com recently noted that thingamabrarians contributed over 13 million tags on just over 10 million books. (Congratulations Librarything!) Users classified books into tight genres. Users did that all for themselves, but we get to benefit. Can't get enough Chick Lit? Get more via Librarything's Chick Lit tagged list.
This is something special.
I predict there will be something equally special appearing on the horizon very soon.