I understand that Google+ users are kindly trying to avoid spamming their friends, but I don't love that the setup is such that I have to specially tell people I want to be in one of their circles to hear interesting stuff. What I would like instead is if people could start their own personal "Sparks", and then I would 'subscribe' or whatever to those. That way, I get more control over how much I can read about my friends' interests in foreign cheeses, or feminism, depending on what suits my interests at any given time.
AND, if I want to post about a topic that people might want to opt in on, I could have a (is Spark the thing?) label/name that I tag to that post to make it a part of that interest group. Sort of like a sub-blog, I suppose.