I think Google+ should have a "Public Circle" option. Here's why::

As an example, if I post a lot on a particular sport (+Nigel Jones e.g. F1) or on coffee makers / music / family photos / mobile phones etc. then I don't mind them being visible to all but I don't want to spam people with things they're not interested in. I could even have a 'verbose' public circle for more regular 'twitter' style posts. However I can't know which topics any given follower would be interested in, but I could allow THEM to see that I have those public circles and add themselves to them.

I wouldn't have all circles public of course but that gives followers the options of which of my posts to subscribe to. This would be a huge plus over twitter/buzz (although I guess being in a circle would prevent them being "public" for searching which isn't good for discovering people, but then I think search indexing should probably be allowed for these 'public circles')

The only concern would be the slightly confusing rules about it, but it would be a 'killer feature' I think - akin to subscribing to a person's twitter hashtag since twitter is an 'all or nothing' affair for following.
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