Tagging files one by one would be a chore, but if I could tag by command-line, I could think of how such a process could be integrated into business workflow.

On a tech site like TR, it would be good to discuss how Microsoft implements this - is a part of the file structure itself, or stashed into auxilliary files/registry? I suspect the former given jeremy's comment that it doesn't work on many file types. I also agree - tagging should be universal or it will just be another ignored feature-bloat.