They specifically added mediocre, hidden filter options and labels in order to kill migration to G+. Once they were sure G+ was not a real threat, they stopped caring about those features they never wanted in the first place.
They had the idea, not the implementation. Circles were not some bolted on feature you had to access through a submenu. They were fundamental.
That covers the “I’m only sharing with this subset” feature. It doesn’t cover the “I want to see what this circle’s members are saying” feature.