In principle, M-Day was an excellent idea: clear the table, get rid of 90% of the excess mutants clogging up the Marvel Universe, reorient the X-Books after a long spate of creative floundering. Consider for a moment just how repetitive so many of the storylines and events of the mid-to-late 90s actually were, how redundant the hundreds and hundreds of faceless cannon fodder minions and nameless conspirators lurking under every rock of the mutant world. Eliminating mutant powers from the vast majority of the world moots a large percentage of these problems. Ideally, the moment House of M finished they should have hit the ground running with a new direction, new storylines, new villains, new directions for old villains, new themes and a streamlined cast.
But that's not what happened. Instead of accepting the new status quo and moving forward, the books rebelled against the idea on the most profound level. This was perceived by some as a passive-aggressive reaction on the part of the creators to M-Day itself, an idea seemingly imposed by editorial fiat. From the moment M-Day hit, the X-Men's major goal was undoing its effects. They took to speaking of themselves as an "endangered species." The books got grimmer and more inward-looking, obsessed with picking at the threads of this one singular moment in franchise history. It's now been five full years since House of M and the books are still obsessed with the resolution of that one storyline.
When I first heard about "No More Mutants," I just assumed the mutant status quo would simply be returning to pre-1990 levels. Up through the end of Claremont's initial run, mutants were still very rare: meeting a new mutant under any circumstances was notable, and every mutant was significant in some way. After 1990 or so, however, new mutants began to show up in simply absurd quantities, often attached to any number of hopelessly generic paramilitary mutant supremacist groups or shadowy government agencies. The problem persisted throughout the subsequent decade until, during Morrison's run, he took the idea to its logical conclusion and simply posited a world wherein mutants had become a significant portion of the population. House of M took this further iteration to its logical conclusion by giving us a supposed utopia wherein the mutant plurality entirely transforms civilization. M-Day should have cut through this untenable status quo and given the creative teams more room in which to move about. In the process they could restored what had historically been one of the book's most reliable engines for conflict and story generation - the process of discovering new mutants, something that had become a particularly uninteresting idea in a world filled with literally millions of mutants. But maybe if mutants weren't quite so common, the stories could return to treating them special.
After M-Day, there was one bit of dialogue I kept waiting to see. You could have had any of the main characters say it, although maybe it would have been best coming from the Beast or Cyclops. Essentially, I kept waiting for someone, anyone, to say "you know, maybe not having so many mutants isn't a great tragedy - honestly, it was all we could do to keep ourselves from destroying the planet many times over. Maybe having the genie back in the bottle isn't such a bad idea. We're all still alive, at least, we're still human, and that's the most important thing." You get the picture: everyone became obsessed with resurrecting the mutant race, finding new hope for the species. It was all they could talk about, the only thing they could write stories about. None of the characters - not that I ever saw - ever actually articulated the idea that having less mutants wasn't necessarily a bad thing.
And this touches on the real underlying problem: the slow-motion car-crash that has been the storytelling "solution" to M-Day has gutted the books in a truly profound way. The reaction to M-Day has made them thematically unintelligible in a manner I don't think many people have yet realized.
Pop quiz: what are the X-Men about? It's simple, even the average American moviegoer knows the answer: prejudice and minority rights. It has been present since the very beginning, even back to Stan's brief tenure. The X-Men have always represented the idea that minorities are first and foremost human beings, and that specific differences can always be overcome by the appeal to larger commonalities. Furthermore, it is accepted as a given that minority communities can and should demand equal rights and representation based on these commonalities. Most thematically-linked X-Men villains were historically split between two camps: human bigots who believed that mutants were less than human and therefore deserved to be segregated or exterminated, and mutant chauvinists who believed that mutants were more than human and therefore deserved to rule or exterminate mainstream humanity*. The X-Men we situated precisely in the middle of these conflicts: mutants were neither worse nor better than humanity, they were humanity.
There was another thematic detail which has long since been abandoned which I also think was crucial to the overall shape of the series: it was for a long time established that two mutants would not necessarily breed "true" - that there was no guarantee mutants would breed more mutants any more than two normal humans who gave birth to one mutant would necessarily breed another. That maybe isn't how mutation works in the "real world" - but this isn't real genetics, either, this is a specific dormant X-gene placed in humanity's ancestors millions of years by 500-foot tall space gods and activated primarily by atmospheric radiation. It may have seemed like a small matter to have, say, Cyclops and Madelyne Pryor's child be a regular human, but it was thematically important because it reinforced the fact that these mutants really were just a part of humanity, no different than people with ginger hair or double-jointed thumbs. Of course, that was all long gone by the time you had Wolverine's kid show up with identical Wolverine powers. If mutants always breed true, then they are one step further to speciation, and if they can be legitimately called a distinctive species, then the civil rights metaphor at the heart of the franchise gets a lot harder to sustain.
But that's not how it is anymore: Cyclops 2010 talks just like Magneto 1980, or Apocalypse 1995. Mutants are a species separate from humanity, they must protect themselves from humanity, they must act to ensure their own survival at all costs. The moment the X-Men started talking like this, they obliterated the moral argument at the heart of the franchise. Reading this latest X-crossover - the supposed climax of all these post-M-Day plot threads - it becomes progressively more clear that not only are the X-Men themselves backed into a corner, but the people who write the books are as well: they need to realize that they've turned the characters from staunch integrationists into de facto separatists.
* This obviously doesn't include thematically nominal villains like Arcade or the Limbo demons or the Brood or any of the Japanese mafia folks who've taken up space in the books over the years - but it's worth pointing out that the profusion of these non-mutant threats was a direct consequence of the fact that the books were always concerned with more than just core thematics, and pulled from a wide variety of genre tropes in order to craft an interesting long term soap-opera serial. This kind of cross-generic fecundity is good for the long-term health of any franchise: it's always good to have a strong thematic core to which to refer back, but never to the extent of making the books monotonous. Think of it this way: if every Spider-Man story were explicitly and solely about power & responsibility, then the books would be unbearably boring - Spider-Man stories can be about lots of things, even if they're all still a little bit about power & responsibility. Same with the X-Men: they're all a little bit about "protecting a world that hates and fears," but ideally the storytelling engine is versatile enough that they can be and have been about lots of other things too.