Phillip Keller asks whether the tagging revolution is stuck, and while rambling through a number of tagging features, posits some ways tagging systems can be improved.
Generally, I'm not a big fan of tagging as categorization. As implemented, one of the best things del.icio.us does is support social navigation, which Keller notes in passing. However, the tail end of his commentary hits on some juicy territory. Most of the tagging systems are pretty poor at creating other things out of a pile of tagged items. Keller's example is creating simple ordered lists. I'll one up it and say a meta-application of del.icio.us would be the ability to create hierarchical outlines by choosing from sets of tagged items. Such an app would be good for doing things like reading lists, wish lists, and playlists.