Typically most modern email clients continue to be static. Why?. Because of security concerns. For instance, virtually none of the standard email clients let you run JavaScript. Now, it looks like Google is loosing up their security concerns because starting today Google+ notifications in Gmail will be a lot more interactive.
Google+ users are already able to add people to their circles from inside the Google+ email. Now Google is bringing the full interactive Google+ experience to messages. Users can now view, comment and +1 posts from their inboxes. Responses from friends will also appear right inside a message.