These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternative Character Interpretation: Depending on your point of view (Or that of the writer) The Empire is either an enlightened, civilized, and honorable nation, a corrupt wreck that's about to fall apart at any moment, or a brutally powerful dictatorship which holds effective control over the civilized world.
Likewise, the Bretonnia can be either the hallmark of feudal chivalry with brave knights and steadfast peasant archers, a bucolic alternative to Empire or a haughty feudal state with snobbish, foolhardy and clueless aristocracy and Dung Ages downtrodden peasants no better than slaves.
In essence, the motivations behind every faction and race can be read in many different ways.
The forces of Chaos, in stark contrast to their 40k counterparts, can actually be considered to be in a gray area, as they have their own deep warrior culture and aren't actually doing anything worse than their counterparts (for a measure), since their raiding and pillaging is completely in-line with their culture's morality. They still worship Daemons and their gods want to destroy the world though.
Author's Saving Throw: With respect to Orcs and Goblins. After Grimgor Ironhide was portrayed as effortlessly beating Archaon the Everchosen into the ground in the official ending story of the Storm of Chaos, Games Workshop had him get involved in the Nemesis Crown campaign, where he was blown to pieces by the very Artifact of Doom he was seeking.
To be fair though, Grimgor blind-sided him and Archaon had already been injured from his fights with Valten and Luthor and various others right before. So it is at least somewhat plausible.
This may also be why Grimgor is still to be found in the most recent Orcs and Goblins army book, and also why the Nemesis Crown campaign was an exclusively online event, rather than being featured in White Dwarf like most of the recent big campaigns (Abaddon's 13th Black Crusade, Albion, the aforementioned Storm of Chaos...).
Broken Base: Pretty much every new Army Book sets off the base in some manner, with certain "exploits" coming to the fore immediately, and some taking a good deal of time to get revealed. 8th Edition completely altered the previous tactics ("Multiple Small Units" or MSU, which featured tiny units, a super-Lord and War Machines to make unrealistic (and unintended by the game designers) but highly-competitive armies — instead large units became a mainstay.
Generally, every new army book will break two bases — the opposing armies will complain about how many great things the new army got, or how their features are better than the features of the players' armies; the players of the new army book will complain about all of the flaws, or how they can't play the way they used to.
A perfect example is the 8th edition High Elves book — Rival players immediately broke into an uproar over how powerful the Frost Phoenix, Book of Hoeth and especially The Banner of the World Dragon (a 2+ Ward Save against all Magical Attacks, and there are a lot of those) were. The book was virtually a Tier-Induced Scrappy (with even many Elf players refusing to use the magic items in particular), yet many High Elf players immediately complained when the Dark Elf book came out with some tricks that were noticeably better than their High Elf versions.
Anytime Matt Ward indulged his habit of retconning previously established lore.
Creator's Pet: Grimgor Ironhide. Many fans dislike him for replacing a better thought out Black Orc character, but writers love him, having him stop Archaon at the last minute rather then any of the characters set up as Archaon's ACTUAL enemies (though this was due to the results of the White Dwarf campaign, not authorial fiat).
Talking about Archaon, he would get nowhere in the Storm of Chaos campaign, had the organizers not opted to make it more "dramatic". Needless to say some on the side of Order complained.
Ensemble Dark Horse: Many given the game's length. To name a few; the Night Goblin warlord Skarsnik, the Chaos champion Wulfrik the Wanderer and the Tomb Queen Khalida. They are special characters who are not the main leaders of their factions, yet feature in stories outside the armybooks.
The Von Carstein lineage of vampires, to the point that the 8th edition army book for Vampire Counts, four of the six special characters were Von Carsteins. This is at least better than the 6th edition (only two special characters; Manfred Von Carstein and Zacharias the Everliving, a Necrarch) and much better than the 7th edition (only the four Von Carsteins).
Scyla Anfinngrim, the only one of the original Champions of Chaos to return in the latest army book, despite being basically a slightly tougher Chaos Spawn with magic resistance.
Evil Is Cool: The Forces of Chaos and the Skaven have some lovely little toys.
Evil Is Sexy: The dark elves, witch elves in particular. And all Slaanesh worshippers/daemons, it's a pleasure god, what do you expect. Although Slaanesh goes quickly from "sexy" to "the Caligula-movie level disturbing" to just plain horrifying.
The new Skaven Hellpit Abomination is sometimes known as the A-Bomb.
The Warriors of Chaos are often known as some variation of 'Heavy-Metal Vikings'.
The Dark Elves have a monster called the Kharibdyss — the ludicrously-difficult spelling all but ensured that within a week, players were calling it "the K-Beast" instead.
Game Breaker: Daemons of Chaos are probably the most infamous example in 7th edition. They're essentially a Brute force/Elitist type army, so one would think that would balance they're powerful units out by their cost. WRONG! Not only was the whole army REALLY strong, but everything had a Ward save (a saving throw against attacks that's usually rare because you it can't be altered or ignored — most armies would usually have them on really expensive units, not everything), everything caused Fear or Terror, and the army's magic was not only powerful, but incredibly under-priced for what it did. All of this basically meant most enemy armies would be running scared or dead before they had much chance to fight back, and even the Vampire Counts and Dark Elves that were considered the best armies after it struggled against them. Current rules Nerfed them, however, with limits place on magic and fear and terror effects getting being nowhere nearly as deadly as they used to be.
Immediately post 8th edition, the Lizardmen became this way mainly due to changes in magic (Slann were allowed an extra die for each spell roll). Ironic given that the book was strong but balanced in 7th edition. The 8th edition Lizardmen book fixed several of the issues.
The High Elf book in 8th Edition features a big one in The Banner of the World Dragon. It effectively neuters many enemy Characters attempting to fight the unit with the Banner (a 2+ Ward Save against Magical Attacks), and basically renders that unit unkillable to Daemon Armies. Even their Frostheart Phoenix is extremely dangerous, packing a -1 to Hit De-Buff on the enemy unit.
Memetic Mutation: A lot from nicknames such as the Hypnotoad for Slann, Hulking Out for when Teclis drinks his strength potion, to things like Sigmar being the Patron God of Shouting and Hammers. Also expect your opponent to declare casting Lord Kroak's spell as "I'MA CHARGIN MAH LAY-ZER!"
Previous editions of the game had Dwarf armies that were quite easy to make overpowered. This led to the term "Beardy" to refer to any army or rule that is overpowered/easily abused.
Nausea Fuel: Ogre eating habits, Chaos mutations, and pretty much everything about the Skaven, depending on one's tolerance for rats; one of the most disgusting aspects of the Skaven are Clan Moulder's "experiments".
Squick: All over the place, from Malekith having his eternally forge-hot armor riveted to his bones to the origins of the Tomb Kings.
Ogre Kingdoms were this for a while, until their 8th edition update made them a common tournament army. Wood Elves also suffered a long lull between updates until late 8th edition.
Villain Sue: Some of the villains in the setting turn into this. The biggest individual case is Grimgor Ironhide, who marches right into the heart of the dwarf territory, and they can't push him out. He marches right up to the door step of one of the four major Skaven clans, and they can force him away. Both cases he leaves just because he got BORED. The only time he lost was against the army of Chaos Warriors gathered by Vardek Crom, and by the end of Storm of Chaos, it's him, not the characters from the forces allied against Chaos, that defeats Archaon, and due to Canon Discontinuity of Storm of Chaos and Nemesis Crown, it means the only times he loses never happened.
Archaon himself has been criticized as this because how the Storm of Chaos campaign completely ignored the results of the game campaign, but that can charted as more the creators fault for their lack of foresight on the campaign would turn out since if it hadn't than the Forces of Chaos would been outright curbstomped and pretty well killed the build up for the campaign.