The staging of the confrontations
I love the way the confrontations between Streep and Hoffman are "choreographed" here. Especially the one where she first accuses him.
I like the way the space is used; how the characters position themselves in it to gain "power" over the other, and to undermine eachother; that window blind. I woudn't say it was very subtle, but it was very interesting to watch, not just because of the verbal back and forth, but because of the spatial aspects of it as well.
In a film that is essentially mostly "people in a room, talking", it's a pleasure to see the visual aspects of it being so engaging as well.