Google+, even though it's several months old now, is in a state of flux. New features are added, irrelevant or underused ones get removed all the time. Just as Google+ Hangouts got a welcomed redesign, the Incoming stream from the Google+ main page has been removed.Chances are, you don't know what the Incoming stream does, even if you're a Google+ user, or you may have not even noticed it. That's part of the reason why it's getting dumped.
Google argues that the Incoming stream was confusing for people, who didn't know what it was for, and was also used very little. It had posts and updates from people that had added you to their circles but which you hadn’t added back, in case you were wondering.
Google has added several ways of discovering new people on Google+, the reason behind the Incoming stream, so it decided to retire it.