If Google built this as "people can send you messages on Google+," and you get a notification in G+, wouldn't that be perfectly normal for a social network? And in addition to that notification, you get an email, saying that "John Smith has sent you a message on Google+," isn't that pretty normal as well? Facebook, Twitter, and LinkedIn all do this by default.

I don't think it's that much different if the from: tag is changed from "Google+ Notifications" to "John Smith" and the subject is changed accordingly. The in-Gmail treatment looks reasonable to me, based on Google's blog post. [edited for clarity]