Before I start ranting about the problems of tagging, you should understand something.
I really love the idea of tagging. Applying multiple tags to a single piece of content means increasing its findability and getting organized in a way that provides flexibility far above that of the folder-based organization method used by modern operating systems.
Using a tagging model, you can associate an email to topics like work, to-do, this_week, and projects. Then you can associate a research paper to library, research, and then to specific topics, such as usability and user_experience. And so on.
The flexibility of tagging means being able to find and organize content in myriad ways, all of which you can quickly define, in any manner ...