I disagree with Maple in that I think everyone in all the houses are passionate---it's what they're passionate *about* that determines the house . . . and that's why choice plays such an important role in Sorting.
Gryffindors: passionate for what they think is the "right" thing. Gryffindors are really concerned about right and wrong, and fairness.
Ravenclaw: passionate about matters of knowledge. Hermione could have been a Ravenclaw, but her passion for right vs. wrong is stronger than her passion for knowledge.
Hufflepuffs: passionate about a person or ideal to which they are loyal. Passionate about staying true to their friends and causes.
Slytherins: passionate about achieving their goals. This is how they end up as cunning and ambitious -- because their goals are what is most important to them.
This is just my opinion, though.