I found Tagging to be an interesting concept and I think it could be useful in a library setting. But, only with a few caveats.
First and foremost, there needs to be a decision about how "Social" our tagging network should be. Opening it to everybody, especially the younger crowd, invites the risk of "Tagger Vandalism". One way of combating this would be to resrict tagging to staff only. However, in a small library like ours, it would severely limit the input.
Two other alternatives that exist are that we could co-opt other bookmark lists. For example, a bookmark list done by Stephen Hawking would be a great place to start for physics assignments. Since delicious is a social site, you can search for specialized networks to "piggyback" upon. Second, we could do temporary tagging or specialized tagging. A mystery book club or anime club would set a bookmark list.
After exploring the delicious site, I have to say that I really like the search engine. You can search by typing the tag and then refine the search by typing in another tag. Visually, it acts as a straight line, one word leading to the next and, if you want to slightly alter the search, you can "X" out the words that you don't want. For example, if you want a recipe for turkey stuffing with cranberries, you type in thankgiving-turkey-stuffing-cranberries. But then you decide that you want to do the stuffing differently and have the craberries by themselves. So, you just "X" out "turkey" and "stuffing" and leave thanksgiving-cranberries.
Simple, but effective.