There's a very odd symbolical dissonance in that Slytherin is the House defined on one hand by cunning and pragmatism, and on the other hand the fact that's it tied so tightly with tradionnalism, "aristocratic power" and Pure Bloods.
JKR's system work quite tightly with archetypal images otherwise. Gryffindor with fire and bravery, is the House of the Warrior caste (those who should be by most Indo-European standarts the aristocracy - they even have the Lion as a symbol) Ravenclaw associated to air and Intelligence fits to the Priest/Scholar class (especially with the way Luna is tied to Death mysteries and certain form of mysicism). Hufflepuff has got earth and harworking and loyalty (and strenght in union) - obviously the Provider's class.
But the Slytherin breaks the mold and disturbs that pretty symbolical pattern.
Because cunning, resourcefulness and disregard for rule is never never the attribute of conservatism, of the people who have all interests to enforce those rules. Cunning is the skill of the Tricksters, of the Puck, of the Loki, the Puss in Boots, the Renart the Goupil. Of the Scapin and Figaro who serve and mock and trap the Aristocracy. It's the skill of the weak folks who have only their wits to triumph (or bring themselves into ever greater trouble), of the ambitious lad who makes a deal with the devil then find a way out of it and makes a fool of the Devil at the same time.
We even say "low cunning" to underline how not noble it is.
And ambition is never the trait of the rightfully powerful people. To have ambition you have to yearn something that you not yet have. You have to want more. For one who has power, there's only to act noble and regal like and not abuse it (and certainly not do "us whatever means necessary) - Exactly like the Gryffindors do.
Yet we find the Slytherin associated with the most conformist and old-fashioned support of power. With people who belong by birth. With extremely rich and arrogant blood lines like the Blacks and the Malfoys... or do we ? Maybe it's because Slytherins gravitate so naturally more toward revolutionnary ideas that they are so many to follow Voldemort (who after all sough to overthrow the WW government in a pretty radical fashion). Except that's not at all how the DE are portrayed.
Then to confuse the matters even more JKR puts all those prankster characters like the Weasley twins and the Maraudeurs and manipulative masters like Dumbledore, Remus and Peter in Gryffindors.
As if the two Houses had exchanged features to make them more similar leaving only the core and very classical moral dilemna between Idealism and Pragmatism. (Watch the Buffy the Vampire Slayer episode the Gift if you wonder what I mean by that)
There's different hypothesis we can make from that.
A Pro-Slytherin "JKR just hates them" attitude would be simply to say that JKR took everything that was good and glamourous about Slytherin and put it in Gryffindor, and everything that should have been bad and rank in Gryffindor to put it in Slytherin. But that's not something I believe.
There's the theory that Voldemort has corrupted what Slytherin stood for and that Slughorn, for exemple, shows a better, purer (how ironic) exemple of the Slytherin ethos. But we have good sources to think that Salazar was just as Pure Bloodist as Voldemort was.
Or maybe this stems from the Wizarding World's siege mentality. That's to say, they had more reasons through the ages to fear the Muggle world than we were let to believe from Binn's homeworks and they saw themselves, indeed, as the weak folk fighting against the more powerful (and aristocratic) Muggles using only their wits and magic. And that's why some founders of that society valued cunning, ambition and pragmatism as good traits belonging to their leaders. They were revolutionnaries who succeeded.
There could be other explanations... I just hope that JKR is aware of that dissonance and that it's got a meaning somewhere in her work.
Any thoughts on the subject ?