I also suspect an Weinergate style mis-post of something that was supposed to be private will occur fairly early on. Though to be fair, nothing is as easy to misdirect as a twitter DM.
On the other hand, I think most people are used to the fact that posting on Facebook is pretty public, and that the Circles feature might give people a false sense of privacy so they are more likely to make that mistake.
Imagine planning a surprise birthday party, where someone +-mentions them, even by mistake. So much for surprises!
As long as G+ doesn't go down this path, I can live with it, but I don't like that a "mention" automatically implies "full access" to whatever has been said, especially given that the choice of whether or not to share a particular post with that person is presumed to be up to the author of the post, not anyone else who was also included.
On private posts, the word "Limited" appears next to the post, on which you can click to see who it's visible to. It's not as obvious as it could be, but it's there.
There's a problem with even being able to see the 21, from the pov of the poster...
I appreciate how Google has brought this into the electronic realm. I do wish there was not a 21 person limit because I already see people sharing with much larger pools and you don't know who persons 22...n are who can also see the discussion.