It's easy to call "Gone With the Wind" racist, but applying the moral views of 21st century America to the society of the 1860s is not the best way to analyze this book.
Reading between the lines of the book, an astute reader will pick up on the fact that often, the slaves are the smartest people in the book. Certainly, Mammy has more common sense than most people. She knows Scarlett through and through, and refuses to pander to her. Prissy isn't a bit more ditzy than the "empty-headed Cathleen Calvert." One is just of a higher social standing than the other.
In the 1860s, people on both sides of the Mason-Dixon Line used the "N" word regularly, and it wasn't considered a bit more polite then than it is now. It was used between friends, among men and boys, and not in front of women or children. Saying it might get one's mouth washed out with soap. It wasn't the pejorative it is today, but it was certainly considered crude and impolite.
Mitchell does not argue for the importance of the KKK, but rather for the regrettable necessity. The South was under martial law and people who were supposed to be U.S. citizens were not given their rights under the Constitution. Violence against Southern women was rampant and the federal law enforcement might or might not do anything about it. It certainly does not excuse the formation of the KKK, but does go a long way towards explaining it, as does the imposition of Radical Reconstruction on the South. Disenfranchisement does ugly things to people, and ugly organizations like the KKK often come out of it. So do ugly laws like Jim Crow. In fact, the horrors that prompted the Civil Rights Movement can be traced right back to Radical Reconstruction and its effect on the South. Because Mitchell so unflinchingly faced these issues, U.S. history is more accurate for it.
I think the novel has retained its popularity largely because it is good storytelling. Mitchell draws her characters in vivid colors and forces the reader to become involved with them. Sometimes Scarlett's behavior is infuriating, but the reader has to agree with those others in the novel, especially Melanie, who admire her for her zest for life and her absolute refusal to stay down for long. She is a survivor and people admire survivors.
People also admire lovable rogues, and no rogue is more lovable than Rhett Butler. He is the ultimate man's man, the openly sensual, sinfully sexy, polished, unrepentant bad boy. He's an alpha male with style and grace and almost every woman, whether she wants to admit it or not, would love to have a fling with Rhett Butler. He's "mad, bad and dangerous to know," and it's that thrill of danger that makes him so attractive.
Who doesn't want a friend as loyal as Melanie, who saw a great deal she probably never wanted to see, but chose to love Scarlett anyway?
The enduring appeal of "Gone With the Wind" is that, for all its 1,037 pages, it doesn't read like such a tome. It moves quickly and keeps the reader turning pages until the very end. A classic novel is a classic because it has those qualities.