Slytherins tend to be ambitious, cunning, and achievement-oriented. They also have highly developed senses of self-preservation. This means that Slytherins tend to hesitate before acting, so as to weigh all possible outcomes of a decision (and how these outcomes would personally impact them), unlike Gryffindors, whose chivalrous natures would likely lead them to react immediately and instinctively.
According to Albus Dumbledore, the qualities which Salazar valued in the students he chose included cleverness, resourcefulness, determination, and "a certain disregard for the rules." Dumbledore noted that all of these were qualities possessed by Harry Potter, who was in Gryffindor.
So what do you think? Am I a Slytherin? There certainly are some rules I disregard, but others I have a very hard time not keeping. And I don't think I have that high a sense of self-preservation. Even with all that, I am a little relieved not to be in Hufflepuff. Not that there's anything wrong with that.