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.
They Wasted a Perfectly Good Plot: Lois had Superman's powers for a day. We could have had a really great issue that showcased not only Lois using his powers effectively but an excellent super-powered team up involving the two.
An Epileptic Tree-ish one about Leo Quintum has become an almost essential part of the story's mythos on the internet after being pitched in an article by Cole Moore Odell. If you've finished the story, check it out.
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.
Crosses the Line Twice: The entire concept of "Bizarro Batman" is so horrifically twisted and wrong that it's hilarious. What is he? He's dead. When he was eight years old, his parents shot him dead in an alley.
Ensemble Dark Horse: Nasthalthia Luthor, despite being a previously forgotten Bronze Age Supergirl villain and making only a few appearances.
Recycled Script: A few moments in this series will seem pretty familiar if you read Morrison's New X-Men. There's a poignant scene involving a spaceship hurtling into the sun, a particularly nasty bald supervillain who eventually gets reeducated by the heroes, a forward-thinking academic who wants to lead human society into a new era, and a story about a future generation of heroes who use Time Travel to stop a villain from conquering Earth. Most noticeably: the series ends with the death of a Messianic Archetype thematically associated with the sun, with that character eventually transcending time and space and traveling back in time to comfort one of the main characters at the grave of a loved one.
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 his aborted Superman 2000 pitch and bring back his 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.
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 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.