Freedom
Now that I think about it, I'll have to agree with Brad Pitts statement about the film in an interview. He believed that the film is about the importance of freedom, and what it means.
In Gone With the Wind, on the other hand, slaves are happy without freedom even though, like Solomon Northup and other slaves, they have been stripped of their right to see their family. That is dumb that slaves can be happy and comical without freedom.