Hazel can add keywords or comments, as well as run scripts, based on predefined rules, but when you said "asks me for a description of what that file is which it then adds to the files metadata" I thought you meant you'd be adding the description as in the Spotlight Comments of the files Info Pane. When I said "annotated", I was anticipating user action.

I do not mean to present myself as an expert on Hazel, in fact I'd never used it before yesterday. But it will definitely take actions on files based on your set of rules. Be careful, because rule-based deleting can happen.