Update: some of the criticism rumbling on twitter yesterday has been neatly summarised by Ian Davis in 'Google's RDFa a Damp Squib':
However, a closer look reveals that Google have basically missed the point of RDFa. The RDFa support is limited to the properties and classes defined on a hastily thrown together site called data-vocabulary.org. There you will find classes for Person and Organization and properties for names and addresses, completely ignoring the millions of pieces of data using well established terms from FOAF and the like. That means everyone has to rewrite all their data to use Google's schema if they want to be featured on Google's search engine. Its like saying you have to write your pages using Google's own version of html where all the tags have slightly different spellings to be listed in their search engine!
The result is a hobbled implementation of RDFa. They've taken the worst part – the syntax – and thrown away the best – the decentralized vocabularies of terms. It's like using microformats without the one thing they do well: the simplicity.
Further, in the comments:
the point of decentralization is not to encourage fragmentation and isolation, but to allow people to collaborate without needing permission from a middleman. Google's approach imposes a centralized authority.
There's also a (slightly disingenuous, IMO) response from Google:
For Rich Snippets, Google search need to understand what the data means in order to render it appropriately. We will start incorporating existing vocabularies like FOAF, but there's no way for us to have a decent user experience for brand-new vocabularies that someone defines. We also need a single place where a webmaster can come and find all the terms that Google understands. Which is why we have data-vocabulary.org.
Isn't the point of Google that it can figure stuff out without needing to be told?