Also there's always issues of spam when you allow site owners to add such tags. Remember how <meta keywords> tag has become useless as its ignored by almost all search engines nowadays.
RDFa, for example, allows much more expressive power than META tags did and lets you get very specific. It also allows you to model relationships you never could have expressed with a META tag. Insofar as there is a lot of value in consuming good actors, there will be an incentive to filter out the bad actors.