The end of the world. Superman fails to save Lois. Then he watches Martian Manhunter burn to death. Aquaman is forced to abandon the ocean as the marine life boils from the overwhelming heat. Green Arrow and Black Canary are left stranded on a building. Eventually, the whole world explodes, with Superman left floating alone in despair. It's a hell of an opening to the story, even if it is All Just a Dream.
The hypothetical defeat of the Justice League and the destruction of Earth in Justice. Everything in that sequence is painful to read: Flash's utter failure to save his city, the gruesome death of the Martian Manhunter, the boiling of Aquaman's oceans, the death of Lois Lane, the crushing of Wonder Woman, the destruction of the Earth and the survival of a tearful Superman who couldn't even lift a finger; even when it was a nightmare implanted by Brainiac and Lex Luthor into their fellow supervillains, it's really crushing how the possibility of such event could be greater than everyone expects, driving them to collectively attack and temporarily defeat the Justice League. Keep in mind, these are only the first pages of the introduction.
When Zatanna and the Martian Manhunter find Aquaman's maimed body. Keep in consideration that Brainiac removed the top of his head and his brain. At first, a worried Zatanna thinks that he is dead, but Professor Caulder (leader of the Doom Patrol) is able to restore his missing parts by submerging him in saltwater.
The almost-death of Zatanna. The chase for Brainiac leads Zatanna to magically teleport herself and her companions right behind Brainiac's consciousness, which is jumping between bodies; it reaches outer space, where Zatanna (who is fully human) fades out from lack of oxygen. Superman basically has to burn through the atmosphere with the beautiful magician wrapped on his cape to make it in time to ground level, and make an enormous effort just to not to blow her up with his super-breath or crush her thorax by administering CPR. He succeeds, but the frantic pace of the sequence and Superman's rush to save her is really painful to watch.
Wonder Woman dying and decaying in the arms of her mother while her mother tearfully begs the Goddesses to help resurrect her daughter from the sand and water of Themyscira. They do.
