I wonder what G+ could have done to pivot after its terrible release. They had some good ideas, and usually Google takes those good ideas of a failed product and rolls it into successful products. Remember Google Wave, where you could work on a document _at the same time_?
Something like the automatic generation of circles in Google Mail, and integrate some kind of generalized wall posts for Google Mail users.
Put a UX team on it with firing authority up to the management level. I deleted my account due to the “<rando on one gmail message> is following you” push & web notification spam which could not be disabled for the first year or so – I heard so many people complaining about that. They had similar problems with sharing where a popular story would be all you saw as it was one entry per person sharing it.
Something like the automatic generation of circles in Google Mail, and integrate some kind of generalized wall posts for Google Mail users.