I don't think Streep was miscast, she was misdirected--or not directed at all. She would have been ideal if a real director was at the helm and reined in her performance. It's a wildly indulgent and unruly performance, which works for Violet Weston but not for Sister Aloysius. In the press for the film, Streep seemed rather displeased with Shanley's directing style, which she described as overly controlling (telling her where to put her hands, how to move, etc.). It seemed like this had the effect of making Streep do the opposite, and in turn we get a performance that is rather hammy. I find some moments in her performance quite powerful. Still, the film belongs to Viola Davis and Amy Adams.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"