Or, “the negative treatment of Slytherin House in canon.”
To me, “the Problem of Slytherin” (which I’ve nicknamed based on the infamous “Problem of Susan” from CS Lewis’s Narnia) is one of the major flaws of the series. Not as bad as my issues with DH, certainly, but something that detracts from the series as a whole.
Again, this is a somewhat unpopular opinion: there are plenty of people who feel that the portrayal of Slytherin was fine as-is, many who argue that it was inevitable because of the books’ first-person Gryffindor-centric POV, and some people who believe that an ambition-oriented House is more likely to produce evil people than one that values intelligence/wit, hard work/patience, or bravery/fame, particularly given Salazar Slytherin’s penchant for conflating greatness and blood purity, making a House that seems pre-made for prejudice.*
*I think that Slytherin’s orientation on blood-purity could have been written as something that was a rational response to his historical period, but I don’t think that JKR intends it to be viewed that way. If mass persecutions were occurring and lots of wizards were dying, then you could make a case for hiding/not accepting Muggle-borns - but again, this is something that’s more grounds for an AU fic based on what we know of Rowling’s Wizarding history, in which witch-hunting/problems didn’t get to be significant until three to four centuries after the Founders’ era, and in which the dangers weren’t that significant in any case. (Yes, that’s no consolation to Nearly Headless Nick, but we’re led to believe that persecutions weren’t that terrible a danger in the great scheme of things. In canon, Salazar Slytherin is not Wizarding Magneto, who believes that wizards need to create a wholly separate society of their own because the real world has given them concrete evidence they’ll never be accepted otherwise - although, again, I do believe that Salazar Slytherin-as-Wizard-Magneto, a man with some valid points but extreme views and methods, could successfully exist in fanfic.)
But, even given the idea that Slytherin held an indefensible, reactionary opinion for his own time, I believe that the books could have - and should have - written Slytherin differently. Because, as it is, there is a huge problem with the general portrayal of Slytherin characters, and, moreover, with the idea of House Unity and that entire theme, which became so important in OotP.
This is long, so I’m placing it behind a cut.
Supernatural fandom: So yeah, we got a five minute promo, several interviews, couple of deleted scenes and a title card. No biggie.
Doctor Who fandom: Poster! Poster! Poster! Fiftieth! Poster! Anniversary!
Supernatural: Hey, anything from Sherlock lately?
Sherlock: *snaps* We're currently going through the fifty second promo to find out exactly where John was eating when Sherlock entered the room and who did Sherlock's hair. Quiet.
Doctor Who: Sorry, man. Hope you get series three soon.
Supernatural: Hey, what's that sound?
*room starts to rumble*
Doctor Who: Is that...
Supernatural: No. It's...it's not possible.
Sherlock: *looks up* Wait, you don't think...?
Harry Potter fandom: *breaks through ground brandishing wands* ALIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIVE!!!!!!!!!