Man, the whole Houses ...er, quadrachotomy (?) was a missed opportunity turned into a suspension-of-disbelief-smashing-sledgehammer; Gryffindor and Slytherin get characterized as only having good and only having evil people in them, while Hufflepuff and Ravenclaw are just brushed off to the sides to make room for their competition.
Gryffindor is the house of Unequivocal Good Guys
TMand Slytherin is the house of Not!zis, and this dichotomy is played up as the main source of conflict the entire series; almost everyone Harry is friends with is a Gryffindor, to the point that Dumblegorp himself, who is practically an Obi-Wan or Merlin expy, was Gryffindor when he was a kid just because he's the Big Good Guy
TM, even though his characterization the entire series makes him seem like much more of a Ravenclaw sort of guy; hell, the entire point of Order of the Phoenix is that Dumblecop is too much of a wimp to tell Harry about his concerns, when the point of being a Gryffindor is specifically one's bravery. Did he bribe the Sorting Hat, or something?
Meanwhile, every villain is Slytherin. Literally
every named evil person in the series is or was Slytherin. Why is Slytherin even still there? Has
anything good
ever come out of it? Why isn't it some kind of litmus test for denying admittance instead? Was Scrumblegort planning on, like, hosting a Slytherin Class Reunion someday and then collapsing the building with all of them still in it?
Then Ravenclaw and Hufflepuff just sorta get sidelined; I mean, at least the very end of Deathly Hallows characterizes Ravenclaw a little when Lovegood gives Harry a tour of their common room, but even then nobody in Ravenclaw even gets noted for being particularly intelligent (the closest thing is that the one remotely important character in it has schizophrenia or something,) it's just kind of offhandedly mentioned as the house of a couple extras and side characters. Hufflepuff is just so utterly generic that even the only "main character" still in it, Cedric, gets like ten or twenty lines of dialogue (admittedly spread throughout the book) before getting offed. I maintain that Hufflepuff is actually just where they put everybody with nothing particularly notable about them to keep the underachievers out of everyone else's hair.
Hufflepuffs are also noted for being prime vampire fodder.