Superman, upon learning that he's dying, merely proceeds to quietly put his affairs in order. Then again, there are quite a few hints he isn't taking it as well as he seems - his refusal to tell anybody about it, and his breakdown while under the effects of black kryptonite, for instance. He is, after all, an absolute pro at hiding his emotions.
The same applies to Lex Luthor concerning his imminent execution. Though in his case, it's very clear his escape was already planned.
Memetic Mutation: Lex's Spiteful Spit at Superman right after the Man of Steel expressed his belief that he still had good in him led to people joking about how that spit was the last of said good. It's the page image for the trope and even the caption there gets in on it.
They Wasted a Perfectly Good Plot: Lois gets Superman's powers for a day and expresses great excitement at the prospect of fighting crime alongside him, but she unfortunately doesn't get to put her powers to much use. Though the chapter introduces two major threats in the form of the Subterranosauri invasion and the Ultra-Sphinx, the former is instantly thwarted by Samson and Atlas' sudden appearance; whereas the latter reduces Lois to a Damsel in Distress until Superman bests the monster in a battle of wits.
Alternative Character Interpretation: Does Jimmy know that Clark is Superman? His lines after Superman blasts Luthor with the gravity gun can either be seen as him genuinely guessing where Clark Kent is, or he's just saying it so that Luthor doesn't realize the truth through his super-hearing.
Alternative Joke Interpretation: The revelation from Bizarro Green Lantern that Bizarro Batman was shot by his parents can be interpreted two ways: Either Bizarro Batman doesn't exist because he was shot to death by his parents as a child, or Bizarro Batman became a bat-costumed vigilante while his parents were still alive and his absence from the Unjustice League was because his parents shot him to death recently.
Crosses the Line Twice: The entire concept of "Bizarro Batman" is so horrifically twisted and wrong that it's hilarious: He doesn't show up with the other members of the Unjustice League because his parents shot him dead.
Genius Bonus: Superman's ancestor Bar-El ends up as the effective warden of Krypton's maximum security prison The Phantom Zone. Scotland's biggest prison is in Grant Morrison's hometown Glasgow, HMP Barlinnie informally known as the Bar L.
One-Scene Wonder: Regan, the troubled teen goth girl who Superman touches down from the ledge, is on only one page, yet that one page is remembered for providing one of the most touching moments in comic book.
Superman saving the suicidal teen, a powerful moment often regarded as one of the greatest Superman (if not superhero) moments of all time.
The streamlined origin retelling is one of the most famous pages of the book, for succinctly summarizing Superman's backstory in four panels.
The scene where Superman cures cancer is up there, since it's an homage to one of the most notoriously silly Superman stories.
The image◊ of Superman flying under the sun, which later was redrawn and used as the cover for the animated feature.
They Wasted a Perfectly Good Character: The series is an absolute distillation of everything great about Superman and his mythos... except for Lois Lane. While the book makes some throwbacks to Silver Age Lois shenanigans like her suffering from Superdickery and receiving superpowers, the book never actually goes into detail about exactly why Superman fell in love with this woman besides vague lip service about "some things [he] just can't help".
Win Back the Crowd: In the sense that the story allowed Morrison to use some of the concepts from their aborted Superman 2000 pitch and bring back their DC One Million continuity.
Big-Lipped Alligator Moment: Quite a few scenes in the middle, particularly the one with Samson, come absolutely out of left field, have no relevance to the plot, and are never mentioned again. While comic readers know that these are whole stories from the comic, they are very poorly implemented into the movie.
Complete Monster: Solaris the Tyrant Sun is a gigantic computer that wanders the cosmos, consuming stars for fuel and leaving the inhabited planets to die. Contacted by Lex Luthor to assist him in his master plan to defeat a dying Superman, Solatis turns the Sun red in an attempt to depower the Man of Steel. Supes counteracts this and leads an army of sapient robots to battle Solaris, but he destroys the army in one blast before announcing his plan to betray Luthor, consume the Sun and force the Earthlings to worship him as a deity or freeze to death in eternal darkness. Superman then tries to feed the Tyrant Sun to a Sun-Eater, but Solaris destroys it. Before Superman destroys the mad machine, Solaris turns the Sun blue to spite Superman by destroying his people.
Genius Bonus: Superman tells his robots to set their receivers to 430 terahertz to be able to hear Solaris. While usually expressed as wavelength, 430 terahertz is the frequency of red light in the electromagnetic spectrum, meaning that Solaris "speaks" with signals in the red light he emits, much like a radio station broadcasts in radio waves that you would be able to hear if you tune your radio to a specific frequency.
Harsher in Hindsight: As mentioned on the main page, writer Dwayne McDuffie died suddenly of complications from emergency heart surgery the day before the film was released on DVD. Made all the more poignant because the film concerns Superman facing his mortality, and he had been giving interviewsabout the film mere days before.
They Changed It, Now It Sucks!: The film changes Solaris' fate from being captured and reprogrammed to being executed (maybe) by Superman, replacing one of Superman's most memorable lines in the process. It also changes the pacing of the prison riot so that Clark fails to save some convicts from the Parasite. The adaptation also cuts two of the comic's most affecting moments; where Superman gets to say a final goodbye to his father, and the famous scene with the attempted suicide.