In other posts I’ve mentioned ongoing changes to the top-right interface, where you can manage apps, notifications and so on.
A recent update is the inclusion of a Settings cog-wheel icon inside the notifications pop-up:
Clicking on this icon opens an interface from which you can enable or disable notifications from different services. At the moment I’m only seeing Google+ listed, but others tell me they’re seeing Google Photos listed too.
It seems pretty likely that Google will add other services here (e.g. Calendar and YouTube), allowing you to customize what notifications you receive.