NPR has a commentary
[audio link] by Robert Franklin, a professor of theology at Emory University in Atlanta explaining what Hillary and Nagin were talking about, and who they were talking to when they made their remarks.
His comments made clear to me why Hillary used the word, plantation, and I now can honestly say that it was the easiest way of getting her point across to her audience. Oh, her point is not what the Republicans assumed it was.
For those too lazy to listen, in the context of the Black church, "plantation" is a place of privilege where you might work, but your existence and opinion are of no consequence to the "elite". For most people, that's a pretty accurate description of Congress for a very long time: if you don't have money or power, they don't care about you.