The added features of email metadata can be achieved without the added complexities of adding metadata to all email, with a simple NLP solution. Gmail already does this with date recognition and google calendar integration, as well as the new email 'categories.' It'll be cool to see it expand so any application can take advantage of it (i.e. key management software and joining a new website).
What would be great is a kind of gmail (or any email client) app store. You give permission to different applications to work on top of your email client to react to different email content. ie Mint/your bank automatically adding item receipts to the card purchase it has access too.
