The interesting thing about the Hogwarts Houses personality theory, from where I’m standing, is that it seems to be based on motivation, rather than what’s strictly considered by the academic world to be personality traits. (It’s interesting because motivation is considered fairly changeable by personality researchers, while traits are more fundamental/biologically based and hard to change. Makes Dumbledore’s I sometimes think we sort too soon that more plausible. And painful.)
Anyway, let me give this a go:
Gryffindors are motivated by what they believe is Right. They’ve got a very clear idea of justice, of the way things should be, and if that isn’t the case they are willing to fight for it. Gryffindors have principles. They will go to war over the things they believe in. But this also means they might just as well become fundamentalists. As someone put it, a gryffindor will happily fuck you up if they believe they’re doing the Right Thing.
Huffepuffs are motivated by loyalty. They put personal relationships above abstract ideas. Huffelpuffs will follow you into battle not because they believe in what you’re saying, but because they are your friend. On the other hand, this may also lead to a my master right or wrong kind of situations, where they stop thinking about moral principles and just trust the person they’ve chosen as a friend.
Ravenclaws are motivated by rationality. They value cold logic and hard facts, and are unlikely to be swept along by passionate speeches or emotional pleas. They’re the type to consider the benefits and disadvantages for all when making decisions. Again, this can be potentially scary, because -for example- Ravenclaws would kill you without any hesitation if they believed it could prevent the death of others.
And finally, Slytherins are motivated by self-interest and ambition. They’re moral relativists, who don’t believe in the great Right or Wrong (the way Gryffindors very strongly do) and wouldn’t hesitate to do things others would consider morally wrong as long as it’s in their advantage. Sounds evil, but it isn’t necessarily so: it means just as much that a Slytherin can be charming and loyal - being hated and despised isn’t exactly a good thing, is it? It all depends on what kind of ambitions they have - and how smart they are, of course.
Seen like that, Ravenclaws and Slytherin are quite close to each other, both being cold rationalists, with the difference that Ravenclaws think firstly of the good of all, while Slytherins think firstly of the good for themselves. And Hufflepuff and Gryffindor are quite close too, both led by their emotions, but while Gryffindors are loyal to ideas of right and wrong, Hufflepuffs pledge their allegiance to people they believe to be worth following.
Put shortly: Gryffindor: belief in ideas; Hufflepuffs: belief in people; Ravenclaw: belief in rationality; Slytherin: belief in themselves.