2 things: Brad Pitt and Alfre Woodard/Mistress Shaw
I'm not saying this was the reason, but did it not come across as a bit self-important that Brad Pitt who happens to be the producer, plays the part of the most anti-slavery white male character in this, in fact the one who helps free Solomon? It feels kind of like when M Night cast himself as the author whose book changes the world in The Lady In The Water. I feel it would have benefited from someone lesser known playing that part, and who may have done it more justice, I might add.
As for Alfre Woodard playing Mistress Shaw, was the accent she used accurate? Because it sounded like it was all over the place wavering from a deep south accent to whatever else there was thrown in there. It was pretty distracting.