I'm very excited to try this out. Context (AKA "Circles") is the biggest feature Facebook still hasn't gotten right. By mirroring the way we think about our social graph in real life, Google is making a huge step toward converging Online and Offline identity. It will be very interesting to see how Facebook responds to this... they might finally have a competitor.
This already sort of happens in Google Reader. Sometimes I comment on something a friend shares, and some random person I don't know will reply to my comment. It's not terrible, but it is unclear and kind of destroys the sense of a private place/conversation.
I think the solution is have discrete "places" instead of multiple lists that interact in increasingly complex and unclear ways.
Edit: this is meant merely to inform, not judge how it is or should be.
With a "Place" metaphor, if you have a private place, nothing would leave that place, and only everyone visible in the place would be able to see any of your content. I think this is just a much more intuitive way to handle groups. It can be more flexible, too, because you could very easily create new places with varying levels of privacy, but the exact level of privacy would always be known to the occupants of the place.
Now imagine the same scenario under the Places metaphor. You go to the "Work Friends" place and share an item. Jill, who is also in that place has access to the exact same list of people that are in the place and so knows exactly who she is talking to when she comments on that shared item. No confusion.
Places, OTOH, is, and I think the metaphor is flexible enough to add other interesting functionality, too.
Edit: Also, can you elaborate on how that works? Is there a list of names attached to each shared item?
I'll quote the relevant part:
> If a post was shared with a limited group of people, you can click Limited at the top of the post to see the profile pics of up to 21 other people who can also see that content, as well as the total number of people on the post.
I would need to test this more fully, but that is my understanding of it.
Twitter is already all about this, and even if facebook started the whole 'like' thingy, they are in a heavy situation. They can't just roll out a whole new kind of content discovery feature, where you met strangers and their content. (think about reddit, hn..)(or could they?) I am not sure, they are so tied to this just my close friends and friend of friends feed, that it would really confuse the users.
The +1 is maybe just a temporary solution, till the time your browsing history will count more. (hence chrome) But the path is pretty clear now what is a social network really good for and how it is used.