I think tagging is very useful. It works much like a keyword search. I particularly like the idea of tagging my own favorites because they are already classified in my mind. It will be very useful to organize sites by tags rather than choosing to put them in only one folder. When I want to bookmark a site I automatically have categories in my mind. The disadvantages are that people may tag the same site poorly making that site hard to search for if you do not know the tags they choose. One should think about tags to make them simple, accessible, accurate and uniform as opossible for all users. I think the recommendation to agree on certain tag terms would be great to do in my school.
I am excited to use del.icio.us. I have previously used backflip.com, but found it to be unreliable, slow, and would just plain crash if I had a class try to access my account. Delicious seems like a much better site and the addition of tags and sharing makes it even better.