Kingdom Come is an extremely conservative work, predicated on the superiority of values inherited from halcyon "Golden" and "Silver" ages, in both figurative and literal senses. The legitimacy of the present time was contingent only on its acceptance of and adherence to the virtues of its predecessors.
On first glance, Earth X would appear to be a retread of Kingdom Come. In actuality, it was: following the runaway success of the latter series, Wizard commissioned Ross to do the concept work for a Marvel version of the same. Although the result was fairly predictable, it was nevertheless enough of a success in fan circles that Marvel felt confident actually producing the book. (In retrospect, it was something of a fait accompli: although his star may have faded substantially as of this writing, there was no bigger draw in the late 90s than Alex Ross, and probably any series he wished to produce would have been seen as roughly analogous to printing money.) But there was one crucial difference between Kingdom Come and Earth X, and that can best be described as a function of the differences between the DC and Marvel universes.
Who runs the DC universe? It's a verifiably Judeo-Christian cosmos, despite the occasional feint in the direction of multi-culturalism. The Spectre is the embodiment of God's wrath. In Kingdom Come, the powers-that-be in the universe are revealed to be the Phantom Stranger, the wizard Shazam, Highfather, Ganthet and Zeus - more or less a bunch of old white dudes roughly corresponding to the image of heavenly wisdom and authority promulgated by Christian mythology. (Ganthet is blue, yes, but essentially still very, very white.)
Who runs the Marvel universe? Well, the origin of the human race in Marvel is that a group of ancient, unknowable and mysterious space gods came to Earth and tinkered with primate DNA in order to create certain subspecies and mutant strains of humanity - for their own never-revealed purposes. In addition to the Celestials, there are also any number of other strange beings - dispassionate embodiments of abstract forces such as Eternity, Death, Order, Chaos - whose attitude to humanity could best be described as "not giving a flying fuck". And then there's Galactus. And then there's folks like the Kree and the Skrulls who see Earth as a proxy battleground similar to the ways in which the United States and the Soviet Union used Southeast Asia, Africa and Central America as proxy battlegrounds during the cold war. And then there's the Living Tribunal, who actually does take an interest in humanity, albeit for the sole purposes of threatening to destroy the Earth if we get out of line. Etc, etc.
Regardless of whether or not Kirby intended for The Eternals to actually be a part of Marvel continuity or no - chances are he didn't - the end result was that Marvel's cosmology became an extremely crowded and extremely weird place. Steve Ditko's abstract mystical entities are in and of themselves pretty damn bizarre, but added to Lee & Kirby's Fantastic Four and Thor, with some heaping spoonfuls of Jim Starlin, Steve Englehart and a few others thrown in the mix, and the overwhelming effect was that the Marvel universe became more or less wholly materialistic in conception, an ethically-neutral environment with Darwin's laws in effect on a vastly incomprehensible scale. Despite a very few nods towards the general concept of God (or, heh, the "One Above All"), the Marvel Universe is basically atheistic. For me, at least, this makes Marvel a much more attractive fictional context in which to spend one's time.
So, if Kingdom Come is about the reassertion of classically Juedo-Christian concepts of morality as filtered through fifty-and-sixty-year-old superhero comics, what's the take-away for Earth X? Essentially, the story is about what happens when the superheroes begin to realize just how much their lives have been influenced by the interference of amoral space gods - down to the very ideas of morality and ethicality. The story begins, in Earth X #0, with Uatu the Watcher narrating Earth's history to a pissed-off Machine Man - essentially putting all the bits and bobs of the Marvel universe's disparate cosmology into one overarching narrative. Although Uatu comes across as self-serving and, frankly, juvenile, the vision of the world he illustrates is strikingly similar to that extrapolated by Nietzsche, particularly in his Genealogy of Morals. The only rational basis for morality is slave morality developed as an inoculation against the self-evident superiority of the supremely powerful Overmen. Through strength of will, unadulterated by bastard notions of compassion, humility and charity, the Overman can take his rightful place as the ruler over the masses of undifferentiated, ethically sick humanity.
In Earth X, all of humanity is sick. All of humanity is infected by the germ of morality, purposefully infected as a means of forwarding the designs of unguessed agents of cosmic significance. The superheroes themselves have been designed as a means of reinforcing the Celestials' agenda in regards to Galactus. On one level, it's all very nerdy and the living definition of "inside baseball": comic-book cosmology barely reaches the level of half-baked, and the Marvel universe, constructed on the fly from dozens of contradictory and willfully strange ingredients, is perhaps the oddest and most half-baked fictional universe in existence. And yet it remains fascinating, for me at least. I admit that since I was a young squirt I have been inordinately fascinated by the cosmic stories, those superhero stories that flirted with epistemological and eschatological significance. Stuff like Daredevil and the Punisher is all well and good, but really, the Silver Surfer and Thor are far more fascinating to me.
As someone who has identified himself as an atheist for as long as I can remember even being able to understand the concept of theism, the idea of imagined cosmology has always fascinated me. And it's something that really only exists in the realm of superhero comics and prose genre fiction. (I'm discounting actual myth, which was not conceived of as fiction and to which it would be doing a great disrespect to refer to as an intentional fiction.) My favorite Tolkien is The Silmarillion. My favorite fantasy writer is Lovecraft. Even though he's not a very good writer by most measurements, I even retain a fondness for Larry Niven's cosmological novels. Urban crime fiction and men's adventure stuff has to be really good to get my attention - but even the worst cosmic fantasy will usually pique my interest.
Perhaps Alex Ross and his collaborator, Jim Krueger weren't quite aware of what they were doing at the time, but in essence they created a vision of the Marvel universe that owed less to the Old Testament and more to the aforementioned Nietzsche. By extrapolating outwards from all the strange fantasy and sci-fi stories that compose the background for the universe, they emphasized the absence of God, and the existence of a post-Enlightenment rationality that supercedes morality. The question for anyone living in our post-Enlightenment world is: how do you create ethics in a universe held together by materialistic determinacy, and furthermore, a society given over to the comforts and delusions of late-stage industrial capitalism? Entertainment is consolation, and capitalistic entertainment (and, let's be frank, all present entertainment is by definition dictated by capitalism) is predicated on repetition and formula. Few genres in our contemporary world replicate the straitened, industrial nature of machine-age production than superhero comics, which are officially produced according to the tenets of atomized assembly-line production. The proclivity of certain social sub-groups to cluster around kinds of genre entertainment which makes a virtue of repetition and similarity to previously defined models - Nerd culture - is a phenomenon that could not exist in any era before the present, an era where industrialized society and the diminishment of want has created surplus leisure time among a sufficiently educated segment of the population. No one for whom daily sustenance is a pressing need can afford to care about Luke Skywalker or Buffy the Vampire Slayer.
So what meaning, if any, do these stories have? What meaning can they have? What do all these elaborate, baroque cosmologies and fictional histories have to say in regards to the accident of their own creation? How do you create meaning in a universe with no contingency for meaning - an empty episteme that can only be approached on purely rationalistic grounds? That is the essential conflict of Earth X: what do heroes do when they understand the very nature of their existence to be empty repetition, ethically meaningless and contingent on commercialistic factors beyond their control or even comprehension? (That's the primary question for people who produce and consume superhero comics, as well.) Well, they set out to do the only thing possible: change the system. Rewrite the rules of reality. There's a reason why the story focuses around a disillusioned Captain America traveling across the country in order to murder a small child . . . and that's also why the book ends with Reed Richards becoming God, a cosmic mutiny in response to the hostility and incomprehension of a mechanistic universe.