The most important thing about Django Unchained is that it's a reaction against, or corrective of, movies like Birth of a Nation and Gone with the Wind. At every turn, it subverts or inverts the racist tropes that have defined Hollywood's—and our culture's—treatment of slavery, the Civil War, and Reconstruction.
The sympathetic, gentlemanly slaveowner? Inverted in the form of Leonardo DiCaprio's venal, brutal, and sadistic Calvin Candie.
The pliant, fearful slave? Inverted in the form of Jaime Foxx's Django, a gifted and confident sharpshooter.
The brave white vigilantes? Shown as fearful and incompetent.
The white damsel in distress, kidnapped by a monstrous black man? Inverted and subverted by Kerry Washington's Brumhilda, Django's enslaved wife who Candie holds as a concubine.
The loyal slave, who cares for and protects his master? Not so much subverted as amplified and made sinister by Samuel L. Jackson's Stephen, who has internalized the white supremacy of his master, and seems to be the one who actually runs Candieland.
There's a whole lot more than this; the fear that blacks would take white women is turned around, as slaveowners in the movie seem far more interested in black women than white ones. The view of the antebellum South as a more dignified era—best exemplified by the fact that people hold plantation weddings—is mocked by Tarantino's deliberate juxtaposition of horrible violence toward slaves with the cultured pretense of slaveowners. Indeed, the movie is at it's most affecting when violence against slaves is depicted; it's never played for laughs, and is meant to provide a visual sense of the evil of slavery (something many Americans still have a hard time grappling with).
I'm still trying to figure out my thoughts on Christoph Waltz's Dr. King Schultz, the dentist turned bounty hunter who inaugurates the story. Of the white characters, he is the only "good guy," but even he is portrayed as captive to white privilege—he buys Django from slavetraders as a way to pursue a lucrative bounty.
As for my verdict on the actual movie? Django Unchained is one of Tarantino's best, with a treatment of race and slavery that is genuinely interesting. You need to see it.
Update: With regards to Waltz's character, a friend notes that he is essentially the movie's "Magic Negro" character, a subtle and hilarious inversion of the infamous trope, where a young white hero is inducted into his quest by an older black man. See: Morpheus in The Matrix, Lucius Fox in Batman Begins, Chubbs in Happy Gilmore, etc.