The most striking aspect of the movie for me was that the violence against the "bad guys" was over the top with fountains of blood squirting everywhere. The violence against the slaves was always off camera or implied without showing it. The contrast served to highlight the realism of the latter in a very effective and disturbing way. I've never felt more uncomfortable hearing the N-word so much as in this movie.
I thought the movie handled an important subject in a film that was simultaneously respectful and entertaining.