Final Crisis saw the return of Barry Allen to the main DCU, as well as the Flash Rebirth miniseries in 2009. Cue cries about how the impact of said character's original death has now been rendered completely meaningless and that this is proof that DC hates change and panders to the Silver Age.
There is a number of fans who called this trope when Young Justice folded into the Teen Titans, and then Identity Crisis occurred, exacerbating the situation. This has led to said fans declaring that most, if not all, of the titles by DC have been stopped as of June 2003. While there have indeed been horrible plots and decisions by DC in later years, declaring that they consider most of DC's Modern Age discontinuity because of that is just bewildering. The first bullet on this folder sums up what the situation really is. Of course, with the New 52 it kinda ended up happening anyways...
Superman: In the 50's the adventures of the Man of Steel started to focus on high sci-fi elements at the same time Superman stumbled upon other Kryptonian survivors like Supergirl and Krypto the Superdog, which earned the ire of Golden Age fans who declared Superman had been "de-uniqued" and therefore ruined forever. In 1986 some of those fans got to reboot Superman, and did so by depowering him, de-emphasizing his Kryptonian heritage and wiping out all other Kryptonian characters... which earned the ire of Silver and Bronze Age fans who liked Kara, Krypto and the high sci-fi concepts. Several years later, some of them had to the chance to write Pre-Crisis characters and concepts back into continuity, prompting Post-Crisis fans to declare the reintroduction of Silver Age bits had ruined Superman... FOREVER.
Marvel hasn't escaped this either, most notably a similar reaction to the aforementioned example concerning the return of Colossus to the X-Men lineup, and even earlier, Jean Grey and the whole Phoenix Isn't Her Retcon (as though not having her being a mass murderer was bad thing, and wasn't all a dream like Dallas).
And the number of people who hate the Scott Summers/Emma Frost relationship to the point of vowing never to enjoy/read X-Men again until Jean returns and gets back with Scott is quite large (which is less ruined forever and more ruined until it gets set right).
M-Day and Utopia have perverted the entire premise of X-Men.
Ever since the Marvel Anime project started, people have been throwing around death threats, cursing Japan for "raping Wolverine". Apparently, the Nazis are also involved.
Though Spider-Man and Marvel have had to weather any number of these moments with varying levels of justification, One More Day seems to have been the mother of them all.
There are some who thought the revelation that Gwen Stacey and Norman Osborn slept together and had two super-powered, illegitimate children completely ruined Gwen's character forever. People also thought the fact that Mary-Jane knew but didn't tell Peter made it all the worse.
One of the worst changes of all is Otto Octavius successfully hijacking Peters body and is now the Superior Spider-Man.
Some fans actually WANT Disney to interfere, as at least then with some corporate eyes listening to the audience, Executive Meddling would, for once, be a much needed requirement for those Running the Asylum.
This complaint is likely due to Public Medium Ignorance. People claim Disney will make Marvel more kid-friendly, not realizing that Disney has owned plenty of non kid-friendly stuff without meddling with it (including the studio Miramax, which has released R-rated films like Pulp Fiction). Or thinking that kid-friendly means sunshine and lollipops. It is possible to find a level that's kid-friendly but still entertaining for adults, as Disney has shown with...pretty much all of their animated movies.
Fear escalated so high, Disney apparently felt the need to include the Paramount logo on trailers and posters for The Avengers. Disney will distribute the movie themselves, but the general public presumably associates Paramount's name with the acclaimed Iron Man and Captain America: The First Avenger, and Disney's name with the potential kidification of Marvel.
The Watchmen fan community had some rumblings of this over the movie, for various reasons.
The 2010-2011 Wonder Woman reboot provoked a reaction like this; partly due to the costume redesign which gave Wonder Woman pants rather than her traditional star-spangled panties (which also introduced a bit of Unpleasable Fanbase, since the previous looks had frequently been criticized for being Stripperiffic), and partly due to the nature of the storyline, which saw Wonder Woman's past continuity rebooted into an Alternate History, removing many of the traditional elements of her backstory and canon. Despite the fact that it was acknowledged upfront that this timeline was an aberration to what was supposed to happen and that the plot was to involve Wonder Woman Setting Right What Once Went Wrong, a lot of fans still complained about how they'd changed everything.
When news broke in 2011 that DC Comics were going to reboot their line of superhero comics back to #1 issuesnote yes, even those series that have upwards of 800 issues by now, the prevailing fan reaction was essentially this trope.
It's only gotten worse with revealed costumes, the confirmed return of Rob Liefeld as artist on Hawk and Dove, and the short character blurbs that indicate that a lot of history is gone forever (the Teen Titans costumes in particular have invoked horror and also parodies).
All that said, some legacies look to have made it through relatively unscathed, the Green Lanterns and Robins most notable among them.
This reboot seems like another nail in the coffin containing the relationship between Green Arrow's first sidekick Roy Harper and his daughter Lian. After the latter was killed in Justice League: Cry for Justice, the treatment of Roy descending into drug-fueled anti-heroism managed to get worse and worse. At first, fans were a little relieved when they learned that Roy would be getting his right arm back in the reboot, then it was announced that Lian would no longer exist.
But the thing that set the internet on fire was the new interpretation of Starfire in Red Hood and the Outlaws. Even though fans have been told it gets better in later issues, they don't care and have dropped the character after the first issue. It's easily gotten twice the backlash of Batgirl, though both controversies died down significantly after a month or two. This is the same with all of the books written by Scott Lobdell, especially Teen Titans.
Serious Internet Backdraft erupted over the guy who "replaced" Thor after Fear Itself, despite the Thor books being the best sellers of Marvel at the time. Also, the guy he replaced hadn't even died in-story at the time the replacement was announced. Also because he and the other 2 replacements (going by the promo material for "Shattered Heroes" aka the November line) looked like they went to the '90s Anti-Hero school. Just when Marvel was easing off the Wangst after Siege! This one turned out to be a major false alarm, as it turned out that the "replacement" was actually an old C-List villain in disguise sent to destroy Asgard from within, and Thor came back in about half a year and killed him.
Some X-Men fans are calling this on Marvel's announcement that, after Avengers vs. X-Men, mutants will be less hated and feared and more integrated with the rest of the Marvel Universe, including at least two new books about the X-Men teaming up with The Avengers. Clearly, this is an attempt by Marvel to tacitly dissolve the X-Men franchise and fold its characters into the Avengers due to only owning the film rights to the latter property. Or something. This was also a false alarm, as the X-Men came out of AvX with more books than they had going in, as well as an arguable ideological victory. The only book that ended completely was New Mutants, whose cast did join the Avengers.
People called the twist at the end of Amazing Spider-Man #698, in which Doctor Octopus hijacks Peter's body and leaves Peter's mind to apparently die in his own broken body "the worst story since One More Day" on the very day the issue was released...the story led to a new long-term status quo and a relaunch for the character that has seen many a person in the book be handed the Idiot Ball. Opinion is still divided on whether the book is the shot in the arm the franchise needs, or the final shot in the head it richly deserves.
Since 2010, Archie Comics has been in dispute with former head writer Ken Penders over characters he created for the Sonic the Hedgehog series. Things came to a head when, in early 2013, two storylines, "The Chaotix Quest" and "Endangered Species" were changed to get rid of virtually hundreds of characters in the middle of the stories. Then came the revelations that the cases had been dismissed and Ken Penders' ballywhooing that he had won back all of the characters he created, that Archie couldn't use them and that he was going to make better stories with them. The altered comics have caused fans to cry foul.
Legacy some were displeased that the semi-good Fel Empire overthrew the Galactic Alliance with help from the Obviously Evil Sith. The latest Skywalker is less than good, and more of a jerky Anti-Hero.
Captain America being a HYDRA supporter has some fans in arms.