Gryffindors are brave, which is useful for fighting oppression...but, you know, terrorism and stuff also requires bravery.

Ravenclaws are curious, which can be used to help people, but it can be used for other things as well.

Slytherins are ambitious, and the story has done enough to illustrate the dual nature of that trait.

Hufflepuffs ... they aren't just one thing. Conscientiousness, Loyalty, and Agreeableness all seem to play a role, but are those things really so strongly correlated? Does this "wholesome" nature finally add up to altruism? Does this altruism extend outside the in-group? I think Rowling was going for some sort of hearty, homespun, down-to-earth archetype there... not sure if it would ever be measurable in a single psychometric variable. If I was writing her story, I'd probably settle on "Loyalty", as in valuing friends and loved ones, with the other two traits just being common behavioral side effects of having this value. In which case, it takes a far-sighted Hufflepuff to extend those feelings of friendship to all intelligent beings...while a near sighted one might become nationalism.