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: There are many fans who absolutely hate the changes wrought by From the Ashes and do not consider it canon, or otherwise dislike other parts of the setting, such as Expedition to the Barrier Peaks.
As D&D itself suffers increasing fragmentation, the debates between Greyhawk purists begin to seem insignificant. (What did or did not happen 20 years ago in an imaginary world is Serious Business!)
Iuz The Evil, the cambion child of the demon lord Graz'zt and the human witch Iggwilv. In his early career, Iuz rallied an army and launched a series of brutal campaigns that gave him the name 'Lord Of Pain'; that or the atrocities he committed on innocents. Eventually, Iuz gathered powerful sorcerers and drained them of life to add their powers to his in a path to godhood. Eventually becoming a demigod, Iuz ended up feared by all those who knew him, with no care towards either of his parents or any in his way. He eventually resorted to murdering his formerly loyal servants and replacing them with Fiends solely for convenience and power.
Likely the most dreaded and feared villain ever to grace the face of Oerth is Vecna, currently the God of Secrets and Lichdom. A once-human wizard who became the most feared of liches, Vecna's evil deeds could - and have - filled volumes. To give a notable example: in his conquest of the city of Fleeth, shortly after becoming a lich, the city officials begged for their lives, offering the whole city and their wealth in exchange for mercy. When Vecna wasn't satisfied with that offer, they offered their own lives. Vecna considered this, and gave one of them, Artau, and his family, to his lieutenant Kas, who tortured them to death in front of the other officials in an ordeal that lasted a day. Vecna then said he was still unsatisfied, and had every citizen in the city slaughtered except the remaining officials, forcing them to watch as he did, then had every victims' head stacked, with those belonging to the friends and families of the officials prominent. He never killed the surviving officials, claiming he was satisfied then, and allowed them to leave the city, saying they'd be under his protection for the rest of their lives - simply so they would live it with the horror they had seen. What truly sets Vecna apart from other evil gods is that no matter how powerful he becomes, he desires more, and like Lolth, cares nothing for anybody but himself. Unlike Lolth, he thinks on a far greater scale, and would unmake and consume reality (killing billions in the process) simply to remake it with himself as the only true god. (This was his actual plan in the epic module Die Vecna Die, but while he failed, the ramifications caused changes in the universe that explained the differences between the 2nd and 3rd editions of Dungeons & Dragons.)
Evil Is Sexy: Graz'zt, Iggwilv, Eclavdra. Maybe Mordenkainen if you go for older guys. (Although Mordenkainen isn't really evil; he just does evil things sometimes.)
Fanon Discontinuity: Many fans hate From The Ashes, as mentioned above. Name something that's set in Greyhawk after Gary Gygax left TSR and you'll find someone that hates it and doesn't consider it canon.
True Neutral: In earlier Greyhawk stories and adventures, a lot of emphasis was placed on some characters' obsession with preserving the balance, especially the archmage Mordenkainen.