I'm sure doctoral dissertations have been done on this book and I by no means pretend to be an expert. This is my humble understanding of this work of fiction.
GWTW is about dynamic, Earth shattering cultural change. It's no coincidence that it was written in the 1920's and 30's when there was also huge cultural changes happening. Instead of writing about the changes brought about by the automobile and the stock market crash, Margaret Mitchell wrote the book as historical reference. This is what happened then, and it will happen this way today as well.
The book has a rather simple plot, which most everyone is familiar with, so I won't go into great detail. The main character is Katherine (Scarlet) O'hara, who is in love with Ashley Wilkes. Ashley marries his cousin Melanie, and on the rebound she marries three times, widowed twice and finally settling on the man who has chased after her Rhett Butler. Their tempestuous marriage finally ends after their daughter and Melanie Wilkes dies, only then does Scarlet realize she's loved Rhett all along, but he's stopped caring and leaves.
The theme is the contrast between the Butler's and Wilkes's. Ashley and Melanie represent the old guard. What life was like before the war. Cousins married cousins to keep the land and wealth within the family, and the marriages were arranged. Duty and filial obligation were what mattered. Acceptance and contentment were what made up their life, love and happiness came with time. The main weakness of the book is that this lifestyle is glorified as an ideal that was lost (gone with the wind). Most of the harsh realities for poor whites or slaves were not mentioned. In fact poor whites were looked down upon as morally inferior and slaves were portrayed as well kept children. In the case of Mammy an opinionated beloved pet (the way your cat or dog is "Part of the family").
Rhett and Scarlet are selfish, egotistical, want to break all the rules, hungry for more, more more, and are fixated on their happiness and love. Scarlet would be happy if Ashley just loved her, and she does everything she can to get him. Rhett would be happy if Scarlet loved him the way she loves Ashley, and he buys her and gives her everything in his power to make her love him. When they both get what they want: for Scarlet Melanie dies and Ashley is free she's not interested, for Rhett when Scarlet finally tells him she loves him the way he's wanted her to, he's not interested.
The Wilkes represent the past with it's rigid moral code of duty and obligation and everyone being in their proper place in society. The Butlers are the future where money is the moral code. The more money and possessions the higher in society you rise. Love and happiness become commodities that are sought after instead of a byproduct of acceptance and contentment. That instead of sharing their lives together like the Wilkes, Scarlet and Rhett merely use each other. Rhett uses Scarlet to mother a child, parade her beauty, display his wealth. Scarlet uses Rhett to restore Tara, show off her wealth to the other women, provide financial security, and keep Ashley financially obligated to her. When they no longer have any use for each other they either argue or ignore each other.
Does our society today reflect Rhett and Scarlet? Marrying for love and trying to find happiness in wealth and possessions. Is our high divorce rate indicative of people using each other instead of sharing lives? Do we look fondly to the past where everything was so much better and they didn't have the stress worries that we do today, but not willing to give up all of the modern technology that gives us the stress and worries.
The movie: All time great classic. Fantastic cast, unusually faithful rendering of the story. No one seemed concerned that Vivian Leigh was a brunette instead of a redhead. That's why she's called Scarlet after all. Still she so captured the essence of Scarlet's character no one really noticed. Clark Gable's greatest role and absolutely perfect for the part. Leslie Howard as Ashley Wilkes was the essence of Noblese Oblige, Olivia De Haviland as Melanie Wilkes the kindhearted, but not naive, Southern Lady projecting her goodness onto everyone around her. Hattie McDaniel as Mammy (the first African American to be given an Oscar) who ruled the roost from a position of servitude. There are many others that have made this a movie withstand the test of time and technology. The Great American Novel is also the Great American Movie.
There's only one part of the book that is left out of the movie and without knowing renders some of Rhett's words and actions puzzling. Scarlet had a son by her first husband, Melanie brother. In the book she ignores him, slaps him, belittles him and turns him into a timid pathetic soul. When Rhett leaves for England with Belle he tells Scarlet a cat is a better mother than she is, he's referring to the way she treats her son and he won't let it happen to Belle. When he comes back he tells her that even a bad mother is better than no mother, it's still in reference to her son. It's understandable that this character would have only cluttered up the movie and had to be left out.
The one part of the book that caught my quirky attention, and was left out of the movie, is in the final confrontation as Rhett is packing and leaving. He asks Scarlet how old she is, and she says 28. The whole book take place in 12 years, she's sixteen at the Wilke's barbeque and of marriageable age. That is a lot to happen in someone so young by the end of the book.
Finally: Grinnygranny made me watch the sequel mini-series off the book Scarlet. That someone else wrote since Margaret Mitchell refused to write one. It was dreadful and I've never bothered to read the book. My main reason is contained in Rhett's final words, "Frankly my dear, I don't give a damn." At that point no sequel is possible because all love is gone, if he ever had love instead of lust. Love is an emotion, so is hate, if he left saying he hated her that's an emotion that can be changed, but apathy, or not caring is an anti-emotion. There's nothing left to salvage at that point. For that reason no sequel could possibly succeed.