But unlike classic revenge tales, where the victim (usually a woman, or a child) is avenged by a hero (a strong white male type), here, Django is not only black, he is also both hero and victim, and one of the kicks of this story is that it is ultimately about the empowerment of the victim.
Django is first introduced to us as one of the insignificant and certainly weak chain gang slaves, wearing a blanket and limping as they are escorted through Texas to their new owners. When he is freed, there is this great slow motion shot at his uncovering his blanket, as it he took away the burden of slavery, showing his back with both muscle (hero) and the scars of the whip (the "chokecherry tree" in Toni Morrison's Beloved). This is the transition from victim to potential hero and the audience is made aware of it in this scene.
While it is now established for the audience that Django is a heroic figure, for him to become a credible hero requires process and a rite of passage. The different stages he goes through are shown by the clothes he wears - from looking like a slave, to looking like a child, and by his social status - from being a slave to playing a free servant. Not quite the heroic type just yet. Very tellingly, Django chooses to wear a (funny) 18th century child-like costume (inspired by Gainsborough's Blue Boy), which may reflect on his immaturity.
The actual rite of passage for Django is not just the killing of the men who tortured and branded him and his wife, but also their whipping in front of the slaves - a very powerful scene where the victim becomes the torturer.
Django's 'heroic' character development continues with the training by the man who freed him and has become a sort of surrogate father - dentist, Dr. King Schultz. He learns how to shoot and of course, he's a natural.
Finally, his appearance changes completely: he becomes the typical cool heroic cowboy and believably so. He is now ready to 'go and save the girl' - this is when the rescue narrative begins.
What is fascinating about Tarantino's stories is how his stories are about empowered victims: a pregnant woman left for dead (Kill Bill), three women getting their revenge on a psychopathic killer (Death Proof), Jews killing Hitler (Inglorious Basterds). There is something definitely exciting for an audience to see a victim taking vengence, instead of having someone else do it for them. There is the feeling that victimization does not have to be a definite status or type.
In any case, Django, just Inglorious Basterds, is on the side of the oppressed.
Django Unchained has a number of types: the smart progressive father figure, the evil slave owner, the evil Uncle Tom, the heartless goal-oriented hero on a mission to save the helpless woman held who needs rescue, and each may represent different facets of the whites and blacks in ante-bellum America.
Sure, Broomhilda isn’t much more than a damsel in distress, but it comes with the genre, (the film is already 2hr45'long).
The controversy over the use of the word 'nigger' seems to me completely out of touch: it is a function of the genre. Should the slave owners use the word African-american instead? This is like saying a film about WWII should not have nazi characters use anti-semitic words. There's a point when political correctness becomes absurd (see this argument developed here).
Of course, from a European perspective, the scope of offensiveness of the slur is hard to comprehend. (It is difficult for instance to understand how, in the land that worships freedom of expression, The Adventures of Huckleberry Finn has been banned from a number of school because of its use of the word 'nigger'. See more here.)
As for Spike Lee who said it was "disrespectful to his ancestors" (here or here), without even seeing the movie - what he means is that no fiction about slavery should ever be made, let alone by a white man. (hence his criticism of Tarantino after Jackie Brown came out). But as he put it himself, Spike Lee does not represent the black community (see here)
Yet, this movie is precisely great for striking a fine balance between comedy and tragedy. Never in the movie, does Tarantino trivialize the pain and suffering of slavery. Whether the story is historically accurate is besides the point - it is a fiction.