Cory Doctorow writes about humans and metadata in Metacrap: Putting the torch to seven straw-men of the meta-utopia.

((Metadata is one of those slippery things to define. I can say it is data about data but does that help? For example, the metadata for a word processing document includes all the title, summary, category and keyword kind of stuff that you are asked to fill in when you create a document, but almost nobody ever does. Dealing intelligently with metadata helps us link things together and make sense of our data in a bigger way than on an item-by-item basis.))

In torching the seven straw straw men, Cory explains how people tend to be lazy, liars, inconsistent, stupid and unable to categorise things reliably. Plus other big human failings. People will never fill in the metadata reliably. It is better to stick to metadata that can be arrived at automatically.