Though universes, timelines, and continuities are often torn asunder in the world of comic books, the most violent of upheavels are often amongst the fanbase.
Let's just say it now. If you take ten random fans of any fandom (for example, 10 in DC or 10 in Marvel, not intermixed between the two companies), and ask them of their opinions about what they like and what they don't like, there is a 90+% chance of them having radically different opinions that clash. It may be safe to say that fandoms of both Marvel and DC are extremely fragmented beyond belief.
The Super Hero comic book medium as a whole is divided as to if the Dark Age was a good thing that brought maturity to the medium, or if it's all just Wangst and the Silver Age lighthearted goofiness was better. The theme of the Modern Age seems to be the conflict between the two groups, both of which are fighting for control of the asylum.
And more broadly, the epic wars over whether superhero comics should be suitable for a child audience or not. Every time something more-than-usually sexual or Gorn-ariffic happens in a superhero comic, some internet smart-arse will create an image macro of the panel(s) with the caption "Hey Kids, Comics!", after which people will point out that superhero comics haven't had a real "child" audience for decades, after which the original poster will say that he knows that, it's just stupid to take a children's genre and make it Darker and Edgier instead of growing out of it like a sane person, etc...
Let's not forget the split between DC and Marvel fans. Then, there's the people into indie comics...
Spider-Man: Brand New Day. Virtually everything about it is either the greatest thing ever, or a bastardization of all that is good with Spidey.
One More Day that has the more universal loathing towards it. Some fans have dumped their hatred on that series alone and decided to judge Brand New Day on its own merits rather than by what led up to it. Of course, there are still fans who say "No! Peter and MJ for life." Then there are some that resent the This Loser Is You aspects of the new "Everyman" version of Peter.
Fandom is split in several factions over various aspects, e.g. many said they would have been okay with Peter and MJ breaking up if it had been done in a "realistic" way, such as a divorce, instead of altering continuity so the marriage never existed. Others note that the post-OMD Spider-Man fails to fulfill some very important aims stated by Quesada to justify the operation - Spidey did not become younger, but actually a little older due to the break between OMD and BND and since "everything but the marriage" still happened, seeing him fall in love can never be the fresh experience it was in the 1960s and 1970s - and that he disappointed the hopes of those fans who after OMD prematurely rejoiced that the Spider-Totem and Sins Past story arcs were now erased from canon.
While One More Day is universally loathed, there appears to be a Broken Base in reference to the two people most associated: Joe Quesada and J. Michael Straczynski. Was JMS disassociating himself from One More Day a noble effort to protest a bad retcon or was it petulant because he couldn't use it to completely rewrite all of Marvel's history? Quesada gets most of the flack for this, but there seems to be evidence that he actually prevented JMS from making it WORSE. The direction JMS is now taking Wonder Woman in isn't helping his case.
This "break" was largely the result of the very public disagreement between JMS and JQ after the publication of the final part of One More Day and to a large extent served to divert attention from the (lack of) quality of the story itself. Straczynski's comments were of course seized on, with people pointing out that the final version of OMD was so bad that JMS, the writer, had seriously considered preventing his name from being attached to it. For those who would have preferred either the marriage to have been left alone or a clean break with a total reboot, the difference between JMS's and JQ's scenario about their wider effects on Marvel's continuity was not big enough to matter (at least in JMS's version, Sins Past would have been erased from reality). Joe Quesada's case also was not helped by the fact that he had already made a bad story worse when in Sins Past, he caused Norman Osborn, not Peter, to be the father of Gwen's twin children, and that the wrangling over OMD - which caused a major delay of its publication schedule - showed that despite having spent years on preparing One More Day and Brand New Day, Quesada and co. had not bothered to think out what changes they wanted to achieve and how they wanted to do it (the latter would only be done three years later with One Moment In Time, as for the former, anything that happened since 1987 is still up to writers' whims so long as they say they think it's not essential, including e.g. Mary Jane's pregnancy during the Clone Saga).
The death and replacement of Peter Parker in the Ultimate Comics definitely polarized the fanbase between those who like newcomer Miles Morales and those who want him gone and Peter back.
The Mighty Thor (aka Marvel's Thor) fan base is as broken as you can get. Half the fan base is divided into those who care only about 'good showings' and having Thor win every. Single. Fight (and they won't accept otherwise), while the other half just wants to enjoy the comics in peace. And don't forget those who hate Beta Ray Bill or absolutely love him, and love the new costume or hate it...
Gambit is another polarizing character from the X-Men: He's either the charming mischief maker and the man perfect for Rogue or a callous Casanova that's an insulting stereotype of Cajuns and Rogue is better off with someone else. He also gets called a Marty Stu.
Furthermore, there is still a divide between Gambit fans who see him and Rogue as soulmates versus those fans who see the Antarctica plotline as the absolute end of any possible relationship.
The Wonder Woman fandom (and to a lesser degree, the Justice League fandom) is rather divided over Diana killing Max Lord before the events of Infinite Crisis. One faction thinks it was a perfectly acceptable storyline plot twist (as Max Lord's mind control over Superman could only be broken with his death) while others considered it both a pointless example of grim-darkness.
Wondy fandom definesBroken Base. There is absolutely nothing we can't make an epic battle between good and evil. Secret ID, or no secret ID? Pre-Crisis or post-Crisis? Like a love interest, or loathe him? (Though never 'her,' there was surprisingly little argument over Io.) Love the costume, or despise it? Yea or nay on the invisible plane? And what about her frigging heels?
Also, her Rogues Gallery. Every so often, there'll be a lot of complaining from one group of fans about how Wonder Woman doesn't have any interesting enemies, unlike Batman or Superman. So, the creators attempt to make some new enemies for her. Another group of fans will then start whining about how the new enemies suck, and why don't they do more with the old ones? Rinse, wash, repeat. This one also overlaps with the Unpleasable Fanbase, since there seems to be a third group who absolutely loathes all of Wonder Woman's classic enemies yet just as strongly resist any of the new ones.
Legion of Super-Heroes. These days, the big online fight is between They Changed It, Now It Sucks fans (and some with more specific reasons) of the original Legion only, particularly the 80s Legion, and fans who accept that there can be more than one version. The first group often claims their fandom is more valid, that their series allowed greater involvement and engagement by fans. No dispute is so vicious as the one between "A Continuity Reboot is never a good option, because it breaks reader investment" and "Rebooting is a legitimate way to replace a dysfunctional series." Legion fandom also frequently argue (in no particular order) whether or not optimism or even utopianism are essential to its future, what the role of Superboy was/is/should be (ranging from "The Legion offers a future setting for Superboy adventures" to "Superboy/man steals the spotlight whenever he appears"), whether the Legion are more interesting as teenagers or adults, and don't get them started on whether "Five Years Later" was any good. Another common dispute goes something like, "Most of the threeboot Legionnaires are unsympathetic!" "Did you actually read the series?" "They are heroes, just with different motivations." "But those reasons aren't what I like about the Legion."
Green Lantern. God help you if you try and discuss who is the best Green Lantern; the fight between fans rallying behind their preferred Lantern is one of the most persistent in fandom. This is parly due to DC ticking off Hal Jordan and old-school Corps fans by getting rid of them when introducing Kyle Rayner as the new GL. Another topic to avoid is why the less well-known John Stewart (a supporting character in the comic books) was made the central Green Lantern in the Justice League cartoons. Charges of racism fly like crazy from all sides of that argument.
Batgirl fans are split over their preference for Barbara Gordon, the original Batgirl, or her Darker and Edgier replacement Cassandra Cain, or her Lighter and Softer replacement Stephanie Brown. Further complicated by the fact that Barbara Gordon fans are split between those supporting her uncrippling and those wishing it happened again. There is always been an Unpleasable Fanbase since Cassandra Cain first took the mantle, and now that 3 fairly popular and quite different characters have had the role, it gets worse, even though all 3 have had successful times under other monikers. Which meant that if DC had simply provided all 3 with a consistent role, they could probably avoid such difficulties, but replacements cause the replaced to be Put on a Bus and ignored. The Barbara Gordon return was made even worse in the noticeable decline of quality, and conscious but unsuccessful copying of several elements of Batgirl 2009.
Part of the reason for the broken base in this instance, is DC's steadfast refusal to uncripple Barbara Gordon and the fact that Cassandra stole her costume from the Huntress, who first revived the Batgirl costume before editorial decided to give the costume to a new character. Also, DC's decision to turn Cassandra evil-evil didn't go over well either following Infinite Crisis, leading to a haphazard retconning of her heel turn to be the work of Deathstroke the Terminator feeding her evil drugs. And now both of those issues have been resolved, the base has broken further.
More accurately, Batman stole the costume from Huntress. Batman and Babs then gave the costume to Cass. Huntress being, at the time and depending who was writing her, somewhere between The Rival and some kind of straw vigilante foe for the rest of the Batclan, this was meant to read as the true holders of the Batgirl legacy reclaiming the role from a usurper and passing it to a rightfully chosen heir. Cass fans consider Barbara's explicit choice of Cass to be a clear sign of her legitimacy. However, this creates something of a second Broken Base with Huntress fans.
Now that Stephanie Brown picked up the suit, the fans have become even more split, especially over whether handing over the Batgirl title from an Asian girl to a Caucasian reeks of Unfortunate Implications or not in the DC editorial offices. Of course, Stephanie herself is no stranger to broken bases, mostly due to her most prominent appearances having Writer on Board or simply bad writing, outside her solo series.
Regardless of whether you prefer Babs or Cass as Batgirl, Babs was well established as "Oracle" and one of the main characters in a book (Birds of Prey) when Cassandra became Batgirl (much like Dick Grayson was well-established as Nightwing and a main character in Teen Titans when DC introduced new Robins). In contrast, Cassandra Cain was in a decidedly bad state when the decision was made to pass the Batgirl mantle on to Steph and leave Cassandra in limbo, which seems likely to poison many more readers against the new girl. File this one under "How NOT to pass on a Legacy" alongside the Jordan/Rayner Green Lantern example.
With the New 52 reboot, DC wound up provoking the base by uncrippling Barbara and making her Batgirl again, leaving Stephanie in the lurch for a little while before being established as Spoiler again. Cass, at least, had previously found a spot as Blackbat, one of the international Batmen, but neither she nor Steph have any mini-series or ongoings in the future. The situation has been complicated further with the fact that editorial considers Steph and Cass "toxic" and has refused requests from writers to have them appear in their titles. To sum up, Batgirl has the most broken base in all comics, with about 5 different factions of thought, which really could be mostly appeased if it was recognized that all 3 are separate, good, unique characters, and each had a home series.
Avengers fans and Brian Michael Bendis fans have basically made Avengers fandom a literal nightmare of a landmine field. Partly because the Bendis fans think Bendis' run is great simply because it's being written by Bendis, while some longtime Avengers fans are horrified at what they see as Bendis "raping" the franchise.
There are also Avengers fans that like Bendis though.
Deadpool. Post-Fabian, either "back to his roots zany-violent awesome," or "homicidal maniac that ignores all character development from Cable & Deadpool."
Of course one has to point out that he was a homicidal maniac in Fabian's run too and he has never been that heroic.
Back to Spider-Man: Most of the J. Michael Straczynski run has this kind of issues. Especially the Sins Past story arc, which reveals that Gwen Stacy slept with Norman Osborn, divided fans between those who sees it as "profanation", those who say its good character development, those who don't care, and those who, because of never-ending arguments in fandom, started to hate Gwen Stacy.
And there is his first story where it was suggested that Peter's powers could come not from radiation, but from magic. Some people say that it ruins Stan Lee's great origin, some says it's far better that Lee's origin, and some say what Spider-Man himself said about it - "I don't care".
Oh, and few believe, that what JMS was trying to say is that Peter's powers are not important and we love him for his sense of responsibility.
Go to a comics forum. Make a comment about anything Grant Morrison has written since JLA (apart from All-Star Superman). Hell, make a comment about something Grant Morrison might get the opportunity to write. Then sit back and watch the flame war between those who loathe him unreservedly and those who're convinced he's the second coming.
Speaking of Grant Morrison, his run on New X-Men is probably the most polarizing series for X-Men fans as some fans claim that it was a fantastic series that changed the status quo of the X-Men while the other half didn't like the way that Cyclops and Jean Grey's relationship was being portrayed and the fact that Xorn is revealed to be Magneto didn't go over well with many fans.
She-Hulk suffers from a ridiculous case of this, given the relatively small size of her following. It all revolves around the different approaches that have been taken to the character. Opinion divides between those that favor the original Savage incarnation, the 4th wall breaking Sensational equivalent conceived by Byrne, the wacky Dan Slott version and the more serious and action orientated PAD version - not to mention the people that consider her transformation to be Fetish Fuel and generally, there is to be no quarter given when it comes to this... And the naming of Lyra (the daughter of Thundra and the Hulk) as the new Savage She-Hulk created a mass case of knotted panties.
Recently Red She Hulk aka Betty Ross is also becoming more and more one with her recent prominence to some fans of the original.
When The Flash: Rebirth mini-series was announced, with the news it was bringing back Barry Allen, the Flash fanbase suffered a split down the center between Allen and Wally West almost identical to Green Lantern's Jordan/Rayner divide. At least this time people could see it coming.
The Barry-Wally fanbase split has only gotten worse. The New 52 reboot has seemingly erased all stories involving Wally as both Flash and Kid Flash, and he's yet to be mentioned anywhere in the new timeline. As a result, the two camps have grown particularly rabid toward each other.
The X-Men fanbase. Its continuity has been runied since 2004 according to some fans (AND Chris Claremont), or the dawning of a much-needed new era with large-scale change for others. Some have flocked over to Claremont's X-Men Forever, proclaiming it the only source of consistency, while others would rather see Matt Fraction screw up the franchise than read Claremont's self-indulgent canon.
There was also a divide in the fan base about which X-Men series was better during the early to mid 2000s, Grant Morrison's run on New X-Men or Joss Whedon's run on Astonishing X-Men. Was Grant Morrison's run on New X-Men more memorable since it changed the status quo of the X-Men and introduced new and intriguing storylines or was Joss Whedon's run more memorable for staying faithful to the original mythos of the X-Men and having great character development among the characters?
Many fans have seem to have taken to the recent split in the Schism X-Men event more seriously than the characters that actually chose sides in regards to claiming loyalty for one side over the other.
The Gray/Palmiotti run on Power Girl. In particular, the more light hearted and humorous tone their stories featured in favor of DC's usual fare. One half of the fandom will tell you it was a great throwback to the Silver Age that finally gave Power Girl a fun and relatable personality, while the other half will tell you it did nothing but cement her status as a one-note character who will never amount to anything other than being the butt of endless boob jokes.
Y: The Last Man got this right after the last few issues. Then there's the seething resentment of Yorick's sister and girlfriend.
The New 52 relaunch already received backlash months before it had occured. Although many complaints are due to costume changes and the like. There was even a protest planned by the more upset fans during the 2011 Comic-Con. This looks to be a rather large iceberg aiming for the DC Titanic.
And yet, Justice League #1 went a seventh printing. Either DC has managed to entice only new fans (unlikely), or most of the people complaining about it still bought the comic. Go figure.
And it's still going on. The newest point of contention is the Earth-2 related books: Is the Justice Society of America forever ruined? Why is DC shoving the Trinity into yet another book? Why is the costume for Jay Garrick so bad? Why did they get rid of Power Girl's notorious boob-window? Why does Earth-2 seem much more grimdark than the already grimdark main DCU earth? And of course, why aren't the Justice Society the same as they used to be?
Once the books actually premiered, only the Trinity issue really resolved itself due to Never Trust a Trailer - despite being the focus of the series' hype, the three were killed off in the first issue.
And of course, the Executive Meddling is still going strong. Some fans are upset that the comics appear to be even more editorial driven than before the reboot.
And then we have what they did to Captain Marvel, now called Shazam, and further retconning Cheerful Child Billy Batson into a jaded cynical teenager who flat out tells to The Wizard that there is no onetruly good in the world, though he has shown good in him where it counts.
What makes this one hurt even worse is that, a year later, DC goes so far as to obliterate the Blue Lantern Corps, the Lantern team fueled by hope. This is made even more rage-worthy when, in an interview with Didio, it was said that "comics aren't good unless they're dark and dreary".
Any time a comic book is adapted to another medium (be it a TV show, movie, video game, or anything else), you can expect more than a healthy dose of this. Typically, one side will despise the adaptation for straying too far from the source material while the other side will praise the creators of the adaptation for not tying their own hands by trying to stick too closely to the source material.
There's the rampant Fanservice character debate as well. One portion, composed most obviously of feminist female fans but also including many males who support feminism or just feel patronised by OTT fanservice, hate the fact that superhero comics are often stuffed with Male Gaze and fanservicey Stripperiffic character designs for female characters, and deem this sexist. Then there are the fans who will argue that male characters are also sexualized or at least physically idealised (if less so than their female pals) and insist that the sexism is completely in their heads. Then, a third group, which includes even people like Gail Simone, that feel that there is sexism in comics, but that cheesecake is a fairly minor aspect of that (instead focusing on the Disposable Women and poor handling of female characters by editorial staff). Some of the first faction argue that fanservice and sexual characters/scenarios aren't universally inappropriate - the real Squick and apparent misogyny comes from fanservice in emotionally inappropriate situations, where superheroines are posed sexily during an action scene, or while being helplessly menaced or outright tortured.
Teen Titans has its own large splintered fanbase, due to the many years of publication and writers' different takes on the team, as well as the group becoming infamous for DC using them as cannon fodder in recent years:
Were the original Silver Age Titans too nonsensical and nothing but a "Junior Justice League"? Were they still a viable team even with those points? With some, you'll see acknowledgments of the first era (even grudgingly), due to the friendship of the original five Titans and the fact that it existed to give inspiration to later incarnations. With other fans, they're more in the camp of wishing that Wolfman and Perez's '80s Titans were the original team with no need for a campy predecessor.
Marv Wolfman's choices in the New Teen Titans are mostly lauded, although another common opinion is the criticism for the title being too melodramatic and wordy. The later portions of the run provided a much larger broken base due to Raven turning evil (for a second time) and committing Mind Rape on former teammates, Cyborg being thrown into a coma and spending most issues as a silent automaton, the introductions of Danny Chase and the Team Titans, and the general darker and chaotic tone.
Terry Long is either considered a character that provided a civilian's point of view to superheroes in marrying Donna, or useless, creepy, and a possible self-insert by Wolfman. Danny Chase encounters similar conflicting opinions from fans, along with most of the '90s team members (in particular Pantha, Baby Wildebeest, Minion, and the Teamers).
Geoff Johns' run has opinions that range from it helping revitalize the Titans and restore some characters for the modern era, to those that think it simply rode on the popularity of the Wolfman/Perez run by simply using a few members but overlooking their history and having them sidelined in favor of the former Young Justice characters. In the reverse, other fans preferred that the younger members would be the only team members and wished that the other Titans would leave. The increase in gore and darkness in the book also divided fans, along with personality changes in the cast (some that were explained, some that were left a mystery). When it came to relationships, Johns pairing up Beast Boy and Raven caused a huge uproar.
In some corners of the fandom, there are intense arguments over whether the original Terra really was that sociopathic and evil, or if she was just the designated villain and if Raven is in fact more criminal and horrible of a character for never being punished for some of her own actions (Mind-controlling Wally West into loving her, attempting to come between Nightwing and Starfire, going evil twice and committing several atrocities the second time around, etc. ). These arguments don't end too well, and usually hinge on which character is the favorite of the fan that adds to the debate.
Superior Spider-Man. Doc Ock stole Peter's body. And lo, the fandom exploded. One portion thinks that this is a great shake up and Ock's murderous Anti-Hero is a better Spider-Man than Peter's idealistic Thou Shalt Not Kill heroics. A fair bit of Draco in Leather Pants is involved, largely because is no longer a fat loser with glasses and stupid hairstyle. Almost everyone else thinks that this is a defilement of all that is pure and good in Spider-Man, produces a thoroughly unlikeable Villain Sue / Villain Protagonist who claims that he's a hero despite having recently tried to kill 99.75% of the population of Earth, and, oh yes, attempts Rape By Fraud on fan favourite Mary Jane Watson.
Another thing that bothers people is no one has noticed that this is not Spider-Man despite his rampantly OOC behaviour and the MU's massive experience with telepathy.
And he was in a room with Rachel Summers, one of the most powerful psychics in the Marvel Universe.
It has so far led to death threats aimed at Slott, mocking comparisons to Knightfall, One More Day and The Clone Saga. Slott's less than temperate response, Trolling Creator tendencies and his main defence of criticism being quoting sales at his detractors as well as trawling message boards and getting the mods on the Marvel boards to defend him/support him has led to him becoming the fandom's number one hate figure, on a par with Joe Quesada.
Talk about any kind of relationships in either Marvel or DC and see the flame wars flare up about which pairing was better for that character, whether it's Cyclops/Emma Frost vs. Cyclops/Jean Grey or Superman/Lois Lane vs. Superman/Wonder Woman.