Confession: I only started reading the Harry Potter books this fall. However, because of my immersion in fandom and Tumblr (and my friendship with certain persons), I was highly aware of the characters, plots, and fan theories. I've known my Hogwarts house for a couple years, as well.
From what I knew of the houses, this seemed pretty accurate to me. I'm ambitious - to a point. I'm pretty judgmental. I'm loyal - to only a select few. I'm determined. I follow the rules - but only if I believe they're right or they're self-set. Slytherins have strong moral compasses, actually, but usually only when it applies to a certain set of rules. (But fun fact: most of us are not racist or bigoted.) I'm pretty darn stubborn and overly emotional. I proudly claim my Hogwarts house, but it frustrates me to no end that everyone, including J.K. Rowling, thinks it's the "evil" house.
Being ambitious isn't a bad thing. Of course, this can manifest differently in everyone. Ambition can mean leadership...or it can mean attempting to eat a whole pizza in one sitting because someone said you couldn't. It means setting personal goals and striving hard to meet them. It means wanting to be in a position of power, but often so you can make positive change happen.
Being judgmental isn't always good. But it can manifest as trying to make choices wisely and sorting out evidence, not just assuming the worst of people and being racist.
We're not loyal Hufflepuffs, but we're still loyal to an extent. I don't like everyone; I don't trust everyone. But I am fiercely loyal and protective of those I choose. I would kill for them, if it came to it. I am hypercritical of any significant others who come into their lives and I would defend them against any bullies. But I wouldn't do this for just anyone; people have to earn my trust and loyalty.
We follow OUR rules. Don't ask me how it works, but there are certain rules I just don't see fit in following. They seem stupid or unnecessary to me. For example, I see nothing wrong with being on my phone in chapel. I do glance up regularly (as long as the speaker is semi-interesting), and I'm usually participating in chapel Twitter, which I argue is a form of engagement. Yet, I believe in being on time to class (or any set meeting/appointment/whatever), in following the speed limit and traffic laws, and respecting other people's property, among other things.
I don't know what Rowling has against ambitious people. Surely she had to be a bit ambitious, to get her book published. I don't know why she couldn't write one entirely good Slytherin wizard. But the fact of the matter is, Slytherin is not a bad house. We can reclaim the house identity and make it better than Rowling ever did.
So don't despair if you were just sorted into Slytherin. We are not the house of Voldemort, the Malfoys, and the Blacks. We are the house of Paris Gellar, Peggy Carter, and Natasha Romanoff. (And, to be honest, Taylor Swift is probably a Slytherpuff just like me.) Now, if you'll excuse me, I need to go add more green to my wardrobe.