Kind of a nice little addition — a new link on user’s profiles to “send a message” directly to that user:
Clicking the link will create a new post, with that user as the only recipient. Note that the ability to do this is not new — you’ve always been able to send a message to a single person. However, this one-click interface for it will likely be helpful for new members.
Also note that private messages appear just like any other. I usually put something like Private Message at the top when I send it, so that users will realize what it is. Perhaps Google should add something to automatically show that if a message has been sent to just a single person.
In any case, nice little tweak Google!