Watching it a second time made me understand more of what QT was attempting, and it helped since I had to shake off the whole 'Inglourious Basterds for slavery' mindset that I had going in the first time. IB was a cathartic what if scenario for how WW2 ended, DU has a lot more heavy lifting to do since the absolute brutality of slavery-era USA isn't something that gets shown too often.
So we get the first half of the film where Django and King do their buddy movie routine, and then (as ilovemovies pointed out) there's a huge tonal shift with the Mandingo fighting scene and Candie's introduction. Of course that's the point because now Django/Schultz are diving head first into the ugliness of slavery when they spent the first half skirting around it.
And through a literal voyage/trip the transition into the 'hellfire' of slavery is completed. And it's why Schultz cracks so early since, as Django says, he's not used to Americans. I originally didn't like the last part since I think Django is a pretty weak protagonist (something I still think is a major issue for the movie) but it's where Django finally becomes a free man (also important is how Stephen knows that the real horror of slavery is how dehumanizing it is which is why he was so insistent on it being Django's punishment).
I'm rambling here but my point is that DU sums up what's so fascinating about Tarantino which is how he wants to have his cake and eat it too. He wants to make a film about a horrific period in America's history and show the brutality of that era, but at the same time he can throw in spaghetti western references and huge ridiculous squibs if he feels like it. It's a high wire act that he amazingly succeeds at for the most part, and touches like selectively choosing the level of gore in certain sequences manage to work without being distracting in any way. DU isn't my favourite Tarantino movie but it might be one of his most accomplished ones. I know people on here like myself (Quentin is another one) have taken issue with QT not maturing as a filmmaker but I've seen huge developments with this and IB.