Franchise Original Sin: This story is the Trope Codifier for the modern overuse of Death Is Cheap in comic books: a story in which the major superhero of the DC universe dies and is brought back over the course of a major storyline. However, back then, killing Superman was actually shocking to the audience because it hadn't been done to death yet (previous examples were rare and usually not planned), created immense Emotional Torque, and the Reign of the Supermen was a brilliant takedown of the '90s Anti-Hero. It's still a classic of comic book storytelling and a point of light in The Dark Age of Comic Books, but it also set such a trend for future Character Deaths that the comic book afterlife had to have a revolving door installed.
Genre Turning Point: Part of the reason that this comic was so important was that, for better or for worse, writers realized that they could kill off a major character and just bring them back later. Starting with this, Death Is Cheap has become so prevalent in comics that it's almost impossible to take any death seriously.
It's worth noting that NO ONE expected Superman to stay dead. Even back then people would tell you that the first rule of comics is "Only Bucky Stays Dead" (the second rule of comics was "Jason Todd isn't coming back either", for added irony when they both came back in the same year). What "Death of Superman" started was killing off/resurrecting heroes for shock value to generate sales. Before this story, superheroes died and came back plenty, but rarely with much fanfare outside of their own titles. After this story it began to happen far more often, and always with huge marketing pushes to make sure you knew it was going to happen.
At the end of the Justice League America tie-in to "Funeral for a Friend," Booster Gold sits by the comatose Blue Beetle's hospital bed and prays for his recovery after seeing the other heroes all paying tribute to Superman, hoping he won't have to go through another hero's death immediately after, especially if it's his best friend. 13 years later Booster really would end up mourning Blue Beetle after Max Lord put a bullet through Beetle's skull in Countdown to Infinite Crisis.
The Toyman's appearance during "Funeral for a Friend" where he's shown to be mourning Superman's death by noting how he always looked out for kids, in light of his next appearance shortly after the whole "Death of Superman" saga ended which controversially re-imagined him into an Ax-Crazy child murderer.
During "Reign of the Superman" a fan commented in his letter that "It's amazing how far comics have come in twenty years. Then Doomsday would have been some monster thrown from Krypton or something". When Doomsday's origin was eventually revealed in Superman/Doomsday: Hunter/Prey, it turned out comics actually hadn't changed that much.
During his fight with Doomsday, Superman muses that he would suspect Luthor was responsible for creating Doomsday if he were still alive. Sure enough, Luthor ended up being the one who created Doomsday in Batman v Superman: Dawn of Justice.
During Adventures of Superman #502 Luthor comments on how he and Superboy are more alike than he thinks. Guess who Superboy later turns out to have part of his DNA cloned from in addition to Superman?
Idiot Ball: Doomsday may be really tough, but he also presented absolutely no threat if he couldn't get within arm's reach of anything. Despite the Justice League having multiple telekinetics at the time, nobody thought of simply picking him up. Granted it probably wouldn't have worked since Doomsday was made unstoppable but points for trying.
It's been pointed out in general that Doomsday and Superman's confrontation devolving into who can punch the other hardest (As Max Landis calls it in his famous video on the comic, "Fratboy logic") is a case of this. In particular because no one really ever tries anything but overpowering Doomsday. It even reflects kind of badly on Superman himself who seems devoid of his own usual smarts for the sake this turning into a punching match.
Like You Would Really Do It: Zig-Zagged. As Max Landis "educational parody film" The Death and Return of Superman explains, comic book readers at the time really did believe Superman was gone for good. At the time, death wasn't something focused on in comic books, especially the main characters dying. As such, when The Cape himself died, it was a big thing for both readers and people who didn't even bother reading comics. However, in hindsight the idea that one of DC's most popular heroes and de factoMascot would really be Killed Off for Real seems ridiculous. Also, after Superman's death and resurrection, every comic book with a dead character decided that it would be equally cool to have them return, until the concept of Character Death in comics was so inherently cheapened that He's Just Hiding! became the constant response any time a popular character died.
Because of the "Death Is Cheap" trope (whose former Trope Namer is "Comic Book Death"), this story is hard to take seriously today. However, it's believed to be the Trope MakerforDeath Is Cheap, so at the time it was a big deal. Heck, it made the nightly news, at a time when Comic Books were believed to be culturally irrelevant.
Lois Lane. Watching the man she loves fighting and dying while still determined to recording the fight is hard enough, but what made her cross into this territory is that, unlike the other heroes and civilians, she is both completely powerless to do anything and, in this storyline, knows both Superman and Clark Kent. While the story's focus is on actions, poor Lois's grief and pain is detailed very clearly, especially with the four new Superman who crop up later in the story.
Lana Lang, the Kents and Supergirl were also this, as they had to keep secret Superman's real identity after he died. Superman was known to be dead, but Clark wasn't.
Jimmy Olsen also deserves a mention as to him he has lost both of his pals and has to watch one of his closest friends grieve.
Alternative Character Interpretation: While still a destructive force of nature, Doomsday displays a few moments of It Can Think. It fights in a more tactical manner against Superman, most notable when it tries to strangle him with his own cape. This could imply that instead of just being a random monster, Doomsday is in fact a sadistic entity who enjoys the suffering of others.
After beating Superman to a pulp, Lois stands up to Doomsday. Doomsday's lack of passion as it attempts to kill her could be seen as showing Lois a degree of respect by giving her a warrior's death.
The fact that after several team movies where Superman has essentially been The Big Guy of the Justice League, with not much else done to flesh him out as a character beyond pairing him with Wonder Woman (in itself controversial), this film is not only actually about him, but finally gives him some much needed characterisation that shows why he's such a beloved hero. The fact that characterisation leans closer to his classic portrayal than his more arrogant persona from the New 52 helps.
After Justice League Dark relegated most of the League to cameos, this movie has the entire League actively involved in the plot. This is especially true of members who haven't been seen active in the League, such as Aquaman, who's up to his third appearance here after doing nothing in his second, and newcomers Martian Manhunter and Hawkman, who likewise did nothing in their first appearances.
For some, the fact that, unlike Superman: Doomsday, the story will be told in two movies instead of one, allowing things to be fleshed out more.
For many who didn't like the Clark/Diana relationship since Justice League War. The Ship Sinking in this movie and Clark finally expressing interest in Lois is definitely this. With many seeing it as a major stepping stone in pulling away from the New 52 style.
The Idiot Ball from the comic version was addressed, as well. Green Lantern is clearly shown using distance attacks and constructs against Doomsday. Doomsday does break out of said constructs, but points for actually trying.
After years of interpretations playing up the "alien god amongst humans" aspect of Superman in comics and movies, Superman's humanity is utilized much more than before through his actions and interactions with other characters, swinging back to the "upstanding good guy with superpowers" aspects that endeared him to the audience in the first place. There's an exchange just after he reveals his secret to Lois that illustrates this return to character perfectly.
After Superman leaves the Flash to clean up his latest act of heroism, Flash mutters to himself that he should join the Titans, then muses that he's probably too old.
When Batman mentions having to speak to Damian's principal, Flash asks if Batman is on the PTA. He follows this up with an impression of how Batman might behave in such a scenario, using the infamous"Bat-voice". Batman's disapproving gaze shuts him right up.
Clark introducing Lois to his parents, and then they proceed to embarrass him by bringing up his past girlfriends, including Lori Lemaris (who's a mermaid). Martha Kent then notes they probably shouldn't be talking about her since she's cooking halibut for dinner.
Despite initially being released the previous year, The Death of Superman got its theatrical debut in January, 2019, mere days before the release of Dragon Ball Super: Broly. As if the Fandom Rivalry between Superman and Goku fans wasn't usually intense enough, the two films feature relaunches of earlier films and sell themselves by way of their villains, Doomsday and Broly, a pair of Unstoppable Rage, Power Of Hate-fueled savages.
Pandering to the Base: Superman enters the opening scene delivering as many quips as possible within the space of a few minutes. It comes across like DC Animated is trying to simultaneously appeal to the Marvel Cinematic Universe crowd and distance themselves from their live-action counterparts.
Slow-Paced Beginning: The movie does take awhile to really get going with the first half establishing the current status quo, Clark's struggling how to tell Lois about his Superman identity and Doomsday's arrival on Earth.
Tainted by the Preview: The fact that this adaptation takes place in the New 52 styled universe, and the apparent dropping of Wonder Woman and Superman as a couple, has been a bit contentious, due to how Lois has had very little screentime and thus development within the films. Some people also weren't fond of the decision to do another version of the Death of Superman storyline, as the movies had already done it before (as the very first film, no less), with Superman: Doomsday.