After escaping Atlanta, Scarlett and Melanie find the land around Tara ravaged, all homes except Tara in ruins, and Scarlett's mother dead of typhoid.
Mammy telling Melly what Rhett and Scarlett have been saying to each other since Bonnie died. In particular (in the movie, at least), when she says that Rhett has been refusing to allow the funeral because, he says, he won't put Bonnie in the dark, because she's afraid of it. Couple this with how much he doted on his daughter and his refusal to allow the 'leave her to scream' policy (at a time when it was common practice!), and it's bad. But then imagine, instead of Mammy's tearful words to Melanie, Rhett yelling it through the door with his voice cracking...
Bonnie's death itself is real sad as well.
In the scene in the movie when they get the casualty lists from Gettysburg there's a band playing a cheerful melody. One of the boys in the band has just found out his brother has been killed and cries while he plays.
That scene is even sadder in the book, where Scarlett is reading the names and realizing that the list includes many of her former beaux and childhood playmates. Her relief that Ashley is alive is eclipsed by her devastation. It's one of many scenes in the book that show that Scarlett isn't completely heartless.
The shot of Scarlett wandering dazed through Atlanta, after the bombardment has stopped, the camera pulling back to show dead and wounded Confederates as far as the eye can see.
Ashley's recital of the Episcopalian funeral service - from memory - over Gerald O'Hara's grave when he realizes that the Catholic service is far too brief for the assembled mourners, who were expecting a much longer and more emotional farewell. Also qualified as a Heartwarming Moment.
Scarlett's miscarriage. She's utterly devastated as this is the first time she's genuinely wanted the baby and cannot bring herself to call for Rhett, who she thinks wants nothing to do with her.
Gerald O'Hara and Bonnie, two people Scarlett loved, dying the exact same way
In the movie, Scarlett's reaction really drives it home
Scarlett: *annoyed* Just like Pa! *dawning horror as she realizes what's going to happen* Just like Pa! *Bonnie falls off the horse and is killed, Scarlett faints*
The ending, when Rhett rebuffs Scarlett's genuine entreaties and leaves her. No matter how much you might dislike her (and with good reason), you realize, this is a woman who within approximately one year has lost an unborn child (the first one she really wanted), her living child, her best friend, and now her husband. That is hard.