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.
Anvilicious: The prologue to The Orc King features a group of murderous anti-orc bigots who go around in hoods and call themselves Casin Cu Calas—the CCC, for short.
Broken Base - Those who like the 4e Forgotten Realms as opposed to those who don't. Or vice versa depending on your point of view. In terms of sales, the group that dislikes the 4e version are "winning", if one can be said to win such arguments. Wizards of the Coast has recently announced that Ed Greenwood, the original creator of the setting, has been given the task of "revising" it for the upcoming 5th Edition, and its already been leaked that Greenwood's first job is to repair the damage done by the 4th Edition.
Base Breaker: Mystra. God all mighty, Mystra. There are two schools of thought on her; those who believe that she is the Realms, and those that believe she is everything wrong with the realms. One of the most popular questions asked post 4e is when will she come back? The two most popular answers are "In my game she never died " and "Hopefully never. " Interestingly, both sides generally agree that bringing her back would be a bad thing; those who were against her death have either given up on the setting already or treat it as discontinuity in their home games, and those who are for her death will have similar reactions to her being brought back.
Myrkul was the old god of death before Kelemvor. While Kelemvor has a firm set of morals and only tolerates the Wall of the Faithless because it's necessary for the continued existence of the other gods, Myrkul was an utter sadist who took gleeful joy in witnessing the agony and suffering the Wall caused. He is stated to have ruled his faith with an iron fist, purposefully leading his followers through sheer fear of his retribution, and executing anyone who defied him, be they man, woman or child, in sadistically brutal fashions, such as burning them alive in a giant furnace. His greatest atrocity, however, was what he did to one of his best priests, who had had enough of the injustice of the Wall and attempted to destroy it. Initially, he just left the priest to rot on the Wall, but just before the priest fully merged with the Wall and attained some measure of peace, Myrkul decides that he hasn't suffered enough and transforms the priest into a mindless being of pure, unbridled hunger that feeds on souls, then sets it loose upon the world, knowing full well the chaos it will surely cause.
Cyric is the Prince of Lies. In the aftermath of the Time of Troubles, when the three prime gods of evil died, Cyric was raised to godhood for his evil in order to replace them. Cyric proceeded to prove his cruelty by murdering and oppressing other gods, sending his servants to steal faithful worshipers from the afterlife to torture in his realm, instituting cruel punishments on the dead. Cyric invented new ways to drive beings made, even succumbing to them himself for a time. Even after regaining his sanity, Cyric gleefully murdered his enemy Mystra, the Goddess of Magic, inflicting the hideous Spellplague that scarred the face of Faerun forever and claimed many lives.
Lolth, the (Demon) Queen of Spiders, is the goddess of the Drow, of spiders and of chaos. Once a member of the Elven pantheon, the Seldarine, and named Araunshee, Lolth betrayed the Seldarine out of growing malice, discontent, and lust for power. She betrayed her husband, the chief elven God Corellon Larethian to his enemy, the Orc God Gruumsh and tried to help the destruction of the Seldarine. When she failed, Lolth fled to the Demonweb Pits and led her chosen race, the Drow, into the Underdark, becoming a twisted and cruel goddess. Lolth would soon trick her own grandson into absorbing the essence of a demon lord to corrupt him into serving Lolth as her champion forever. Lolth had the Drow establish a society absolutely built on sadism and backstabbing, solely for her own pleasure, with cruel edicts consisting of demands for the sacrifice of every newborn male child at birth and demands for her priestesses to carve out the hearts of males they grow too fond of. What sets Lolth apart from other evil Gods is that she cares nothing for her race or her worshipers, viewing them as merely tools for power. For her own amusement, she will capriciously withhold her favor from loyal Drow just for the pleasure of seeing them die. Lolth has nothing less than genocidal fury towards all surface elves and routinely directs her followers to murder them. After becoming more powerful, Lolth proceeded to purge the Drow pantheon of other deities, ending her victory by destroying her own daughter, Eilistraee with savage glee.
Rivalen Tanthul, one of the 12 Princes of Shade, the remnant of the empire of Netheril, is the most powerful of his brothers. While his father and siblings are certainly ruthless, they genuinely love one another. Rivalen is the Nightseer, high priest, of the Dark Goddess Shar and murdered his own mother to instill his family with the loss needed to turn them to Shar's worship. Rivalen mentally tortures his younger brother Brennus with the vision of the murder and mocks his father, High Prince Telamont, about loving Rivalen too much to stop his mad plans. In the vision, Rivalen's dying mother simply asked Rivalen to hold her hand so she wouldn't be alone, to which Rivalen calmly denied her request to savor her suffering. Rivalen engineers massive casualties and Shade's takeover of the lands of Sembia, savoring the bitterness and pain of those he betrays. A century after Sembia's takeover, Rivalen attempts to finish Shar's manifestation, revealing his ultimate goal: to die and take all that exists in Toril with him. Defined by his cruel nihilism and utter lack of regard for any living being, Rivalen sought deaths on scales that not even Gods such as Cyric would attempt.
Counterpart Comparison: Some of this with Mask and Vhaeraun—both gods of thieves, Mask is the Shadowlord while Vhaeraun is the Masked Lord and the Shadow, Vhaeraun is canonically mixed up with Mask on the surface, and the tail end of third edition gave them both limb loss, mother issues, and death.
Evil Is Sexy: Asmodeus is described as being the most handsome of all devils.
Beshaba also counts.
Averted with Lolth part of the time. In the retcon of her background developed for the Forgotten Realms, her original form after being cast down by Corellon was that of a bloated, hideous spider with an elf's head. But she frequently uses shape-shifting to invoke the trope.
Fanon - Discussions and interpretations taking place on the Forgotten Realms sections of the Official Wizards of the Coast D&D Messageboards
Fanon Dis Continuity - Many fans choose to ignore the developments in 4E, mostly on the grounds that the 4E setting is less an update and more a new setting. 4E arbitrarily replaced or laid waste to large stretches of the world-as-it-was, used a Time Skip to get rid of most of the mortal cast of characters, and killed off a large number of prominent gods, including Helm, Mystra, Elistraee and almost the entire drow pantheon.
Game Breaker - Some consider various high-level NPCs to be Game Breakers.
Shadow dragon Aurgloroasa "The Sibilant Shade". Loves raiding in a shape of some rival dragon, who then receives dragonslayers. Spied upon a dwarven city 12 years before using their high priest as a puppet next 33 years and in 20 next years built with dwarven hands the temple, then got it consecrated to her deity (she's a priestess of dragon death god) and killed everyone in the city. Later she goaded the Cult of the Dragon to "find" and "seduce" her with riches exorbitant even by their standarts, only to get the ceremony subverted and turn into dracolich without their usual controls.
Cyric. Cyrinishad debacle, Trial of Cyric, Mystra's death, Helm's death, Tyr's eventual death, Kezef's freedom, the Spellplague...Yeah. Bastard indeed.
Mary Sue: The Seven Sisters. In addition to having a buttload of character levels, being Chosen of Mystra gives them special benefits such as immortality, a 10+ bonus to constitution (negating the biggest weakness of arcane spellcasters), extra spells and immunity to one spell of each level, as well as access to the versatile Silver Fire ability. In particular, official profiles for Alustriel and Storm tend to emphasize that they have no personality flaws whatsoever, always make time for the little guy, and are beloved by everyone they encounter. Storm is even said to be so favored by Mystra that she's saved her from death several times, but to emphasize how special she is, Storm is said to not expect this help and to not think of herself as better than others because of it. Oh, and according to Ed Greenwood, they don't get fat when pregnant.
Roddy's morals were always certainly ambiguous, but his villainy isn't clearly established until he strangles Kellindil with his bare hands.
When Cyric slashed the tendons in a fleeing soldier's ankles and when he ruthlessly killed Midnight's halfling friend. The first could be forgiven by simply saying he had to do it to keep his cover, but the second is unforgivable.
Jarlaxle manipulated events to start a war over the port of Luskan. After the fighting was over, with tons of citizens dead and the city partially destroyed, he made sure that food was scarce by preventing the flow of supplies into the city, and starved the people of Luskan until they were ready to rebel against the new establishment and install Jarlaxle's associate as the next ruler. As Affably Evil as he may be, it's difficult to forgive him for this. The fact that one of the people whose death he causes is Deudermont does not make it any easier.
Then again, it's Luskan. A major pirate nest and power base of both Arcane Brotherhood and Kraken Society. Not quite Zhentil Keep, but not very far from this.
The death toll from the initial war was stated at 4000, with hundreds more dying from the elements after the fact. It's explicit that a good portion of these deaths are civilians and children caught in the crossfire.
Worth mentioning is that this isn't treated as a Moral Event Horizon in the actual book. Jarlaxle skates by, suffering no real consequences for it. He's called out by Drizzt, but more specifically for the death of his friend Deudermont as opposed to thousands of others that died during the conflict. The kicker? Jarlaxle lies to Drizzt, saying he didn't arrange any of it, just that he hopes to take advantage of the situation. Drizzt drops it, and while it isn't made clear whether or not he believes Jarlaxle, what is clear is that many of the fans do.
And then she eventually crosses the Despair Event Horizon in Hand of Fire and commits suicide. Convinced that she accidentally fried her true love, Narm Tamaraith, with a spellfire blast. (Hint: She didn't, and Alustriel had to lie her silver-haired hiney off to keep Narm from jumping off a cliff right after her when he finally woke up.) Great way to end the story, Ed. Really.