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.
Broken Base: The system is loved and hated, often at the same time by the same people.
Canon Sue: In one of the earlier campaigns, Ulrich Kiesow introduced his favorite character, the Sorceress Nahema as this. She haunts the setting since then, and is usually enigmatic and just better than anything a PC could ever achieve. The whole people of the kobolds might fit here as well.
Many of the authors are guilty of this, introducing their personal wish-fulfillment PCs as uber characters with stats that were impossible to achieve by normal means, even if a player would constantly roll the highest values on any given dice.
Ensemble Darkhorse: The sheer volume of NPC has led to quite of these. Many of the villains are more popular than their 'good' counterparts.
Game Breaker: Few and far between, but they do exist. Some of the truly terrifying ones are highly situational, though.
A guild wizards staff can be used to store spells which can be used instantaneously. Given enough preparation time, this can roughly double the amount of spells he can cast before he is out of mana and circumvents casting time and gesture or word requirements.
Taken Up to Eleven with the Applicatus spell, which allows for spells to be temporarily stored and released when certain conditions are met. Some GMs ban the spell outright, many others at least add houserules to cut down on exploits.
Magic-users' main limitation is their all-too-quickly-exhausted stores of astral energy. However, they can learn to Cast From HP, and the healing potions an experienced alchemist will brew in large batches even while on the move can restore any character to full health instantly...resulting in mages with effectively inexhaustible "ammunition".
All "full" magic users can learn a ritual to burn up their entire remaining life force to fuel terrifyingly powerful spells for a short time, after which they irrevocably die. However, they can also learn a spell that lets them swap bodies with another person's, and the extra power from the ritual all but guarantees that this spell will go through. As a result, the druid who has just nuked half a continent is probably already safely inside another's skin while the poor bastard's soul is stuck in a body about to die. This tactic is actually acknowledged by the writers, one of the major evil NPCs has been using it to prolong his life for centuries.
However, that particular spell is generally a blacklist of sorts, except for Druids, where most of the exceedingly powerful spells (like all the spells dealing with manipulation of time etc.) are. These spells are generally intended to be for NPCs only and there are almost never official ways (like deriving it from a spellbook, learning it at an academy etc.) to get a hold on them - not to mention the side effects like antagonizing the God of Time, for example. A GM who hands out spells like these probably has a totally different idea of gamebreaking mechanics.
Because there are so many spells that can be modified and work out differently in different representations (Druid, Guildmage, Witch, etc.) a few spells can be incredibly powerful if used right, which led to the hilarious example of a player who, using only mechanism available at character generation with no extra Game Master grace given, managed to max out the effects of one spell far enough to create the fantasy equivalent of an atomic bomb.
The fact that magic users aren't explicitly denied by the rules to wear heavy weapons and any type of non-metal armor (with some exceptions like druids, who mustn't use metal at all) opened a lot of loopholes for Munchkins. The 4th edition however introduced a currency-based leveling system somewhat comparable to The World of Darkness that encouraged specialisation and made "multi-classing" a lot less attractive.
It Was His Sled: Emperor Hal was a woman. The twist ending to an old adventure module, this information has no further relevance to the setting. Despite this, many fora have a ban on this information.
Mary Sue: The setting has its number of writer favorite NPCs who are strictly better than anything a player could ever achieve. One spectacular case is the Sorceress Nahema, the favorite NPC of the founder of TDE. He died almost ten years ago, but his creation still haunts the setting.
Nightmare Fuel: Many of the demons might qualify. The demon queen of rape and her servants certainly do.
For reference the ones who only might: the demon king of tyrannic rule and law (Going way past Knight Templar), the king of axe crazy, butchering warriors, the queen of everything scary in the sea (channeling Cthulhu and other nasties), the king of strife and unfaithfulness, the queen of undead and nightmares, the king (or queen) of illusions and madness, the king of black ice and hunting every possible prey (including Hunting the Most Dangerous Game), the queen of mutation, the king of greed, the queen of sickness and filth, and the king of perverted fire, air, stone and earth along with perverted inventions.
Rooting for the Empire: In the early days of the Dark Eye the slave-holding city-state of Al'Anfa was the source of all evil and scourge of the continent's south. Then Villain Decay set in and The Empire was thoroughly pummeled whenever it reared its head (and sometimes even when it didn't). Nowadays the fanbase is nearly unanimous in its support for the battered city.