Mammy was black, but her code of conduct and her sense of pride were as high as or higher than those of her owners. (2.5)
Many of the black characters in the novel are obsessed with social class, and see their status linked to that of their owners. Having the people on the bottom of the social scale enthusiastically endorse the status quo is supposed to make the status quo look okay; if the folks on the bottom are happy, what's the problem? It also makes it appear that Mammy is part of the family, connected to Scarlett by love rather than because her labor and her life have been taken from her by force.
This might be a good moment to mention that in Alice Randall's The Wind Done Gone, a parody/sequel/critique of Gone With the Wind, Gerald sleeps with Mammy, and has a daughter by her. Relations between masters and slaves, often involving outright rape or some form of coercion, were fairly common. Even being literally part of the family (half-sisters and half-brothers) didn't change the class status of black people in the old South—and in fact could just serve to emphasize it in particularly painful ways.
[…] she respected them, and, in time, learned to admire the frankness and forthrightness of these people, who had few reticences and who valued a man for what he was. (3.98)
Ellen admires her neighbors at Tara for not being as class-conscious as her own family. The idea that they "valued a man for what he was" is ludicrous, though. The first thing Ellen's neighbors see is not whether a man has value, but whether he's white or black. This line is a lie—and, in this vein, the whole novel could be seen as one long, vicious lie about social class.
Of course he wasn't a gentleman and there was no telling what men would do when they weren't gentlemen. There was no standard to judge them by. (9.139)
Rhett isn't a gentleman since he refuses to abide by the rules of class. This makes him dangerous and disturbing; the novel as a whole doesn't like war and revolution and people getting out of place. Or does it? The old South, where women couldn't even eat for fear of looking un-dainty, also comes across in the book as restrictive and stultifying. From that perspective, Rhett's freedom from class makes him exciting and sexy. He can do anything.
"Nothing, no, nothing, she taught me is of any help to me! What good will kindness do me now? What value is gentleness? Better that I'd learned to plow or chop cotton like a darky. Oh, Mother, you were wrong!"
She did not stop to think that Ellen's ordered world was gone, and a brutal world had taken its place […]. (25.91-92)
Scarlett is unprepared for hardship because she grew up so rich, basically. She was at the top of the social pyramid, but the war has overturned that. Order (of social class and hierarchy) is replaced by brutality—and brutality is defined to mean that white people have to do the things that they'd formerly made black people do.
Scarlett did not realize that all the rules of the game had been changed and that honest labor could no longer earn its just reward. (31.24)
The point here is that after Reconstruction, Southern white people were not free, and their labor could be taken from them at whim—as when the taxes are raised on Tara and Scarlett is in danger of losing the land. The suggestion is that before the war, virtue was rewarded, and honest labor gained honest pay. This neatly erases the fact that the entire economy of the South was built on refusing to give black laborers their just reward.
She wanted to feel superior and virtuous about Belle but she could not. If her plans went well, she might be on the same footing with Belle and supported by the same man. (33.36)
One of the major class divisions the novel is interested in is that between good women and bad women—virtuous Southern Belles and prostitutes (in this case, named Belle as well). The war undoes this distinction, as it undoes many others.
"Class?" said Scarlett, startled at the idea. "Class? What does class matter now, so long as a girl gets a husband who can take care of her?"
"That's a debatable question," said Old Miss. "Some folks would say you were talking common sense. Others would say you were letting down bars that ought never be lowered one inch. Will's certainly not quality folks and some of your people were." (40.77-78)
Part of the reason Scarlett is able to do so well after the war is that she doesn't really care about class; she's willing to work, and pursue money, and forget about what's supposed to be right for a person of her standing. The suggestion in the novel is that this is in part because she's the daughter of Gerald, who was a self-made man himself. Which in turn, suggests that even her lack of care about class is a function of class; she's the daughter of a hustler, and she's a hustler, too.
He felt that it was a traffic in human bodies on a par with prostitution, a sin that would be on his soul if he permitted her to do it. (41.185)
Frank is very upset at the idea of using convict labor; he sees it as "traffic in human bodies"—it's not classy. But owning slaves was fine. The problem here isn't the traffic, but the fact that you're treating white convicts the way black people were treated. The lack of class is precisely the messing with class distinctions based on race.
"I did so want to see you! You must come in the house."
"I can't do that, Miz Wilkes." Belle Watling's voice sounded scandalized. (46.47-48)
Belle, like Mammy, is shown to be a good person because she supports the class system that treats her like dirt. And, by the same token, her support for the class system shows that the class system is a good thing. It's a fantasy of a world where the people you step on are just begging for you to step on them. Which is appealing if you're someone who identifies only with those doing the stepping, but for everyone else it's more than a little nauseating.
"You should have insured a place for your children in the social scheme years ago—but you didn't. You didn't even bother to keep what position you had. And it's too much to hope that you'll mend your ways at this late date. You're too anxious to make money and too fond of bullying people." (52.74)
Rhett is angry at Scarlett for not working to make sure her class position is intact and instead relying on money alone. This is somewhat unfair to Scarlett, it seems—especially since part of the reason that making money is scandalous for her is that she's a woman. If she were a guy, her get-up-and-go would be seen as pluck, rather than as dangerous.