Fanfiction being what it is, and with fanfiction writers' predilection for making their version of the source material at least a bit darker than the original, it should come as no surprise that most fanfiction writers' portrayals of the world of Harry Potter have a lot of darker sociopolitical atmosphere. In even the mildest, there is an assumption that the Potterverse is rife with prejudices, bigotry, and archaic caste systems brutal in their medieviality.
Not to deny that there is obviously some of these things in the canon material (the mistreatment of house elves, Umbridge's loathing of non-humans and non-purebloods, etc) , but I think the fanwriters, for the sake of drama, are steadfastly ignoring any contrary signs. The wizarding world cannot be nearly as racist or bound by pureblood privilege as the fanwriters like to portray it.
First thing to consider: hate groups are made up of those who believe themselves disenfranchised. The IRA's ranks are filled with irishmen harboring bitterness against the British crown, not lords and dukes sitting in Parliament. The KKK did not even exist until AFTER the Civil War, and was made up of white men who blamed the blacks for their downfall. If Wizarding England was as feathered a nest for purebloods as most fanfics portray it, Voldemort would never have filled the ranks of the Death Eaters.
Second thing to consider: Taboos. Draco Malfoy may spit "mudblood" with every other sentence, but early on it was established that this was regarded as so outrageous a vulgarity that Ron Weaseley immediately went for his wand when Draco used it. Compare and contrast the usage of the word "nigger" in the United States today. The word is so verboten that people--- even in supposedly racist "Deep South"--- actually have to bolster their courage to say or even type it, in even a purely academic setting. You don't get a taboo like that unless the sentiments behind the word are already in exterme disfavour.
Third, you don't get nobility going to school and mingling casually with commoners. When the working class nouveau riche in Great Britain started moving into upper class circles due to the rising wealth of the Industrial Revolution, and started putting their children into higher education, children from "commoner" families underwent a gauntlet that makes the worst of Hogwarts look benign. The sons of Lords would not be rubbing elbows at the same dinner table as the sons of textile merchants. Yet in Hogwarts there is no caste division at their tables or their dormitories--- divided by house and year, certainly, but caste doesn't even make an appearance. Except at the Slytherin table. In fact the Slytherins are effectively a dumping ground for all the pureblood prats whose families STILL want to make a big deal about their lineage, while the rest of the houses intermingle "upper" and "lower" classes indiscriminately. The very existence of Slytherin, once the house of the cunning and ambitious, now demoted to the House of the Inbred and Isolated, shows how far into disfavor the worldview of lineage and caste has fallen.