Tagging

Tagging is a very common feature in various database type applications nowadays. It's very tempting to try and use it as a hierarchical or disjoint categorisation tool, but this is obviously flawed! I have tried to use tags as such, but it doesn't work well, and breaks the real idea of what tagging is about.
Tagging is supposed to be haphazard and instinctive. If you scatter many tags everywhere, and use the tags, you will naturally settle into a vocabulary that unlocks a lot of extra information from your database, whether you're using a personal or a social tagging system.
A small number of well chosen tags that has the effect of causing a hierarchy or a disjoint categorisation reduces your flexibility in choosing tag names in the future, and makes things needlessly constrained.
As concerns social categorising systems, maybe we need parallel systems: tagging, hierarchies, sets and more. The more ways we have to link up data, the more useful it is I would say!