It may be too early to express opinions about something so new as Google+, but in the short few days I was playing with it, I started to like it a lot. Many bloggers consider it to be a Facebook killer - and it (unlike two previous Google attempts on social networking) eventually may become that. Conceptually, however it is much closer to Twitter than Facebook.
The biggest difference is that in G+ there is no peer relation - "friendship" that needs to be confirmed by both parties. Both ends are disconnected in Google+ - set of people whose updates you see in your stream and set of people that see your updates is not the same and can be in extreme case disjunct.
In G+, like in Twitter, everybody chooses set of people whose updates you want to see - you "follow" people. This following does not require any approval on their end. Unlike Twitter, you can easily organize the people you follow into groups (circles) and with single click limit the news stream only to members of this group (where one person can be in many groups). It is like having multiple Twitter accounts, each with different set of people followed and switching identities.
On broadcasting side, you can select the target audience for each post by addressing the message to be visible publicly (Twitter default), by any member of your circles, your extended circles (kind of "friends of friends"), one or several named circles, or even named set of people from your circles. Whether your message will actually appear in their message stream depends on whether the addressee did add you into their circles. Twitter does allow you only public updates. (Yes, ok, there are "protected tweets" by who uses them ?). Facebook allows you updates only for all your friends or single FB group.
For example, I have group named Following with members like Andy Ihnatko, Leo Laporte, Leo Babauta and other internet celebrities. Clicking on the circle will show me all public updates from these people, but it would contain none of the posts made e.g. by Andy for his circles - of which I am not member of. Similarly, should I post an update for the "Following" circle, Andy would never see it in his news stream (despite of that I do have him in my circles) as I am not member of his circles. This way Google created incredibly flexible framework that can deliver any mode of communication available from Twitter and Facebook:
1) Twitter following: see above
2) Facebook friends: both parties add the other to circles
3) Facebook groups: named circle (plus possibility of multiple groups and administration that does not suck)
4) Friends of friends: extended circles
5) direct message to person (in your circles): update with single person recipient
What G+ does not have (yet) are applications/games and public developer API. I am pretty sure that promised availability of the later will cause the explosion in the first. It will be very interesting to see the impact of this new, much more flexible "trust model" on the kinds of applications available.