As an aside, related but not directly relevant to my previous post about Slytherin House, I think there’s a tendency in fandom to overcompensate for Slytherin’s portrayal in the books by romanticizing both the Slytherin characters and the house itself. It’s the old “attraction of the bad boy” principle. Then this spills over into other fandoms when fans Sort those characters into houses.
To give an example: I’ve seen a lot of Harry Potter-Sherlock crossover fic that sorts Sherlock into Slytherin, and I think that’s a wrongheaded choice. Sherlock might be described as cunning, but he’s not really ambitious. What he values, to the near-exclusion of all else, is the Work–that is, the use of his mind. The fault, real or perceived, that he hates most in others is stupidity. To my mind, that makes him an obvious Ravenclaw, and he’s also a good example of some very Ravenclaw-ish flaws: inability to admit when he’s wrong, dismissal of people seen as less intelligent, and a lack of work-life balance. (John, now, is a harder call; I can’t decide between Gryffindor and Hufflepuff.) However, Sherlock also has my best example of a non-evil character who’s a textbook Slytherin: Mycroft Holmes. One doesn’t become the British government without being tremendously cunning and ambitious, but by all evidence he’s done his best to manage the country responsibly.
Come to think of it, Artemis Fowl is also an excellent example of a Slytherin type who makes good. He’s a genius, but he’s much more interested in ways to leverage his genius for personal gain than intellectual achievement for its own sake. In later books he channels his efforts into more altruistic purposes, but there’s still ambition there, and that’s not a bad thing; I suspect Slytherins are better suited than Ravenclaws to run big corporate interests like Fowl Enterprises.
Another example I’ve thought of recently is Cabin Pressure‘s Douglas Richardson: intelligent, yes, but primarily as a schemer. Martin is much more of a book-smart Ravenclaw, whereas Douglas gets by on charm and cunning. Arthur, bless him, is unquestionably a Hufflepuff. I’m tempted to sort Carolyn into Gryffindor by default just to complete the quartet, though I think you could make a case for Slytherin.
(For what it’s worth, I’ve always identified as a Ravenclaw. But I agree with JKR that Hufflepuff is really the most admirable.)