Take, for example, Gypsies, a game book that tried to replicate the Victorian trope of the mysterious, magical gypsy... in the 20th century. Apparently, no one realized it was stupidly racist until it actually went to press. Most White Wolf fans just pretend it never existed.
The Vampire: The MasqueradeSourcebook known as Dirty Secrets of the Black Hand presented the True Black Hand, a secret society of vampires devoted to fighting "Soul-eaters", spirits that were infecting the rest of vampirekind. Fans scorned the entire concept as a complete 180 from the game's mood, dubbing the idea "Vampions" (a portmanteau of "vampire" and the superhero game Champions). White Wolf eventually took advantage of a major event in the World of Darkness, the cataclysm surrounding the rising of the Ravnos Antediluvian, to wipe out the True Black Hand, and subsequently revealed that the group got everything wrong.
To make the "Soul-Eaters" bit even worse, they were alien space parasites and in every way appeared to have been swiped whole cloth from Necroscope. The Tzimisce had always appeared to be a mashup of Necroscope's Wampyri and Dracula, but Dirty Secrets of the Black Hand's Soul Eaters made it pretty clear things with Vicissitude WERE the Wampyri with the Serial Numbers Filed Off (but still not as nasty as actual Wampyri). This was even more obvious because there was a Necroscope RPG by West End Games released a year before Dirty Secrets was published. Both the Necroscope RPG and WoD system used D10s for their dice, so while not using identical systems, it wasn't particularly difficult to port one system into the other (and with the timing, may explain what happened).
When it comes to Dork Age moments in the oWoD nothing can beat Sam Haight. Originally written as a mistreated kinfolk (one who has werewolf blood but can't change forms) who skinned five werewolves in order to become one himself and became corrupted by evil, White Wolf decided that it would be a good idea if he became a main villain for the rest of their gamelines as well. As a result he became a horrifically overpowered vampire-werewolf-ghoul-mage who had powers from all the game lines at the time. After he was finally killed there was talk that he could come back as a super powered Wraith, at which point the writers said that he had his soul forged into an ashtray, and was promptly ignored save for a single line about a screaming ashtray in one of the last novels.
For Werewolf 20th, the writers took advantage of the 'metaplot agnostic' nature of the setting to reset Haight to his original portrayal, the werewolf skin-thief, dumping everything that came later.
Warhammer 40,000 2nd edition is considered somewhat of a dork age. Rampant character cheesiness (read: overpowered), vortex (black hole) grenades taking out dozens of regular troopers, and some extremely poor model design combined to make it something rather ill-remembered.
Most veterans found Warhammer 40,000 3rd edition a big Dork Age, where they stripped the setting of almost all of its background with pamphlet sized army books, and dialed up the grimdark in the little background that was left while simultaneously dialing down the clever things about it. On top of this, the rules were oversimplified to the point where in each army book they had to introduce more and more special rules to differentiate the different statistics which were previously represented with something like a single number (like movement). Also, characters became MORE cheesy in 3rd edition than 2nd edition.
On top of this, they introduced the Tau (who many at the time felt didn't fit) and the Necrons who were inserted as a sort of Mary Sue race. Luckily the Tau have been made to fit better in recent times and the Necrons are not so infallible anymore. Perhaps ironically, it was the introduction of the Tau and Necrons that heralded the beginning of the "3.5 edition", where GW started trying to fix 3rd ed's problems.
Unfortunately, the Fifth Edition seems to be going through a Dork Age for the Space Marines. Mostly due to the horribly bad writing, utterly ignoring the most basic parts of the canon and incredible favouritism towards the Ultramarines. Also repeatedly invoking the Worf Effect upon the Sisters of Battle, scaling down their force and repeatedly featuring stories of them being horribly butchered. All of this can be blamed upon a single author, Matt Ward, who has fans screaming for Games Workshop to fire him.
Thankfully, at least THAT part of the Dork Age is over.
The 2013 version of Codex: Tyranids is by many considered to be this. The writing is not particularly bad, but most of the lore is simply recycled from the former book with barely any new information, in the edition it was released in (the 6th) Tyranids were unable to ally up with any other faction leaving them at a huge disadvantage and the rules were also generally considered bland and uninteresting. Worse yet, many special characters and other things that made Tyranids a worthwhile faction (such as Spore Pods) were outright removed without any sensible replacements. Many a Face Palm was given.
After FASA was shut down, ownership of BattleTech passed to WizKids. The result was an attempted a continuity reboot of sorts with "Mechwarrior Dark Age" under the WK "Clix" system. The net effect of this was to piss off a lot of long-time players, who dubbed the game "Dork Age".
Fortunately for Shadowrun fans, though, they handled that license pretty well.
Some fans considered the Shadowrun products and novels to have suffered a Dork Age when they kept being infiltrated by cross-promotion from Earthdawn: a phenomenon that ended after FASA licensed the latter to another company.
Many fans have had a similar reaction to the changes to the setting introduced by Catalyst Games, the newest owner of the Shadowrun license, who have insisted on trying to 'update' the game world by making it more like 2010, drifting away from its 80's cyberpunk roots.
With that being said, the reaction could be seen as both Fan Dumb and They Changed It, Now It Sucks; by releasing Dark Age, WK Games kept Battletech alive after FASA's demise and paved the way for the revival of the game under FanPro and later Catalyst Games labs.
Part of the problem had to do with a serious lack of communication. WizKids strongly promoted Mechwarrior: Dark Age while simultaneously not promoting standard Battletech early on, giving the impression that they were killing the original game in favor of Mechwarrior. Then, when the Dark Age game was actually released, fans found that none of the original factions were represented fighting in something called "The Republic of the Sphere" which was built singlehandedly by some new character after most of the original factions were nearly destroyed by the Word of Blake, a techno-religious organisation. This, naturally, turned out to be quite a turn off to many older fans, who didn't note that it was All There in the Manual in fiction readily available on WK's website, in the tie-in novels and the information in the game itself, and what was depicted was Republic propaganda. The original factions were still very much around and the Jihad hadn't actually been quite as bad as it was originally portrayed, though it was a pretty major event - it's just that many fans simply assumed the worst without bothering to check the facts.
The less said about Mega Traveller and Traveller: The New Era, the better. Games Designer Workshop (not to be confused with Games Workshop) decided that somehow it was a fantastic idea to set fire to one of the best-realized sci-fi settings in all of gaming. Players stayed away in droves, and GDW paid the price when it went bankrupt several years later.
Tellingly, all of the editions produced after The New Era (and there have been at least six, depending on how you count them) are set either well before the Rebellion/Final War or in an alternate timeline where it explicitly didn't happen.
If you want to talk about specific sets listed here, well...
Before the ban list, Dark Crisis was not well received, due to the lack of "good" cards and the mehness of the Archfiend archetype.
Cyberdark Impact is almost universally mocked for being full of useless cards, including the unfortunate putting-down of the LV monsters with Allure Queen and Dark Lucius.
Tactical Evolution is similarly reviled due to its limited useful card pool and the lukewarm reception to the Gemini monsters (as well as being the set that introduced the "TCG-Only" cards that helped started the UDE Dork Age above).
For the more "elitist" duelists, most of the GX sets are classified as this, for three reasons: First, the over-pimping and at the time impractical Elemental Heroes, all of which fell down the bottomless trap hole. Secondly, the over-reliance of Monarchs. The reason why the Monarchs were overused was because of the third reason; because they (aside from Cyber Dragons, certain Destiny Heroes, the Horus and the Armed Dragon sets), were one of the few series of cards that were practical to play, even in the aforementioned decks. Many of the GX cards were either too pyrrhic to use or suffered from a case of over-specialization, sometimes both! Consequently, many players used the monarchs as a means of getting a powerful monster out, keeping field presence, disrupting the enemy, and applying pressure to the opponent without having to spend an arm and a leg in either costs or maintenance. Monarchs lead to the biggest case of Complacent Gaming Syndrome that Yu-Gi-Oh had ever endured for quite some time.
The Gold Edition sets weren't very well received, either, thanks to them being generic reprints of rather old cards with the "gold" rarities that the sets were named after only adding a little foil along the edges.
There are valid complaints about each one: Original D&D (edition 0) had all the flaws that the first edition of anything could be expected to have. Advanced D&D (AKA 1st Edition) failed to fix many of them. 2nd edition was needlessly obtuse and complicated. 3rd had extreme variation between power levels in classes, as well as some cumbersome skill rules (there were separate skills for hiding and moving silently). 4th edition suffers from notoriously poorly written math (-4 to -7 on a roll of 1-20 is a pretty devastating gap written into the core rules), reverse power creep, as well as feeling a bit too much like a video game. And yet D&D (any edition) has been the best selling Tabletop RPG in print throughout its history, except for a few years in the 90s where it was outsold by Vampire: The Masquerade, and 4th edition, which has been overtaken by Pathfinder, a third-party continuation of the third edition. So now a fifth edition is being worked on, which is preemptively being considered a Dork Age by some.
With the course the setting will take in the upcoming 5th Edition still not being completely known, the "Post-Spellplague-Era" of the Forgotten Realms seems to be held in this regard by many fans. Advancing the timeline by over a century, killing of many of the popular characters and deities, and all but annihilating most of the less popular regions of the world, it is a very different setting that does have its own merits, but significantly different from its 20 year history.
Likewise, 5th Edition of Paranoia. The writers were less interested in the Black Comedy of the setting and more interested in taking blatant, unfunny potshots at other gamelines. (The only supplement released was a Take That at The World of Darkness.) Once the slate was cleared, 5th Edition was officially declared an "Unproduct", and all discussion of it deemed treasonous.
Magic: The Gathering had two genuine Dork Ages, during the Urza's and Mirrodin blocks. In both cases, a handful of Game Breaker cards led to a tournament scene in which all decks in Standard (the primary tournament format, using only cards from the past 2 blocks and most recent base set, making for a low barrier to entry compared to formats where $500 cards are essential staples) were either "Obvious Top Tier Powerhouse.dec" or "Counter-Deck to Obvious Top Tier Powerhouse.dec." Both blocks led to extensive bannings (which Wizards of the Coast only does when absolutely necessary, under the logic that selling a product and then refusing to let the customer use it is an excellent way to discourage the customer from repeat business). In both cases, the issue was exacerbated by the powerful blocks being followed by underpowered blocks, meaning that the dominant cards stayed dominant for the full two years of their legality, rather being displaced in their second year. While the years since then have not been free of cries of "They Changed It, Now It Sucks," those two were the most prominent banning cases nigh unsurpassed until now. Even though 2011 saw the banning of a few cards in Standard tournaments, the number of cards banned are relatively minor.
The Urza's Saga case was definitely the more severe of the two, having earned the nickname "Combo Winter" and resulting in the only case of a card being emergency banned in Standard (In other words, banned before it was even released). Mark Rosewater, current head of Wizards of the Coast R&D, notes that throughout his entire time at the company, the Urza's block was the one and only time that the entire R&D department was called up the CEO's office and yelled at.
On the opposite end of the scale is Homelands. The game was still Growing the Beard and didn't have a real plan for designing and releasing sets on a regular basis. Short on designers, they let some of the storyline people design a set. The result was a set that was very flavorful, but severly underpowered and lacking in new and interesting mechanics. Things were made worse by the fact that it came out during a year with only one other set of new cards (Ice Age), the previous year had seen two relatively lackluster sets released (The Dark and Fallen Empires), and it would be another nine months until another set was released.
Some fans might also name the Kamigawa block, which directly followed the original Mirrodin block. Based heavily on Japanese mythology, it basically removed all the basic monster archetypes and replaced them with very similar but differently named monsters, e.g. instead of goblins there were akki. Given that people quite like theme decks, this might be regarded as a bad idea. The set was also underpowered in comparison to the previous set, leading to a very stagnant Standard format still using mostly older decks, and the new, fairly insular mechanics didn't tie in well with older blocks. It's less disliked now, in non-Standard formats, but at the time it wasn't a well-regarded set at all.
Ars Magica has third edition, which introduces the realm of Reason. It makes a certain logical sense, in that Ars Magica has always had vague ties to White Wolf and the old World of Darkness (the Tremere vampires began as House Tremere of the Order of Hermes, most famously), and Reason aligns nicely with the Technocracy where the Order and the Realm of Magic align with Mages. The problem is, magic in Mage and magic in Ars Magica are pretty different, with Hermetic magic being primarily a scholarly art, with skills and systems devoted to copying books of both arcane and mundane knowledge, and the quality of a covenant's library being a major contributor to its overall rank and reputation. Introducing Reason meant that, in areas with a high Reason aura (particularly libraries, y'know, places full of books just like the ones Hermetic magic depends on), all magic in the area took penalties proportionate to the aura, and generally changed Hermetic mages from wizened scholars ever in pursuit of knowledge, to a bunch of insane, magical weirdos whose powers not only depended on ignorance, but were actively rejected by logic and intelligence. Prior (and later) editions kept the aforementioned magical weirdos to Houses Criamon and Ex Miscellanaea where they belong.
Reason also trumped the other Realms (Faerie, Divine, and Infernal), the idea being that the supernatural can only exist where people can believe in it, and Reason brought the metaphorical skepticism shotgun to the paranormal knife fight. The problem is that the setting is called Mythic Europe. Faeries, angels, mages, and demons are all equally and easily proven to be real. Setting the system up for Reason to negate supernatural power was a lot more like denial or outright delusion. In practice, this was a lot like having your angry landlord come to talk to you about your rent, and reacting by shutting your eyes, covering your ears, and chanting la la la la la, can't hear youuuu!, thereby making him disappear, even though you're still in your apartment and you still owe rent.
Gamma World's 2003 "Sixth Edition" is regarded as the Dork Age of Gamme World, mainly because they ditched most of the outrageous cheeziness of previous editions and tried to make it like a serious and sober RPG. It was not generally well received by fans, most of whom loved the cheezy fun of the setting.
Exalted: You get one of two answers depending on which side of the Broken Base you contact. Either most of second edition was the Dork Age, with terrible rules and poor writer communication, or the Ink Monkeys/coming third edition is the Dork Age, with a different tone, the setting being heavily revised and new splats added. This has produced many knife fights.