I completely agree. I have thought a lot about this, because I feel that Slytherin has been given a unfair reputation. Everyone here has brought up a lot of good points, the main one being that the books are told from Harry's point of view. Harry has a dislike of Slytherin because of his problems with Malfoy, Snape, and - most of all - Voldemort.

Harry is, in a sense, the super Gryffindor. Always charging in places without thinking, ready to be a hero. The story is shaded through Harry's eyes to make Gryffindors look faultless ans Slytherins to look evil. We know that this is not the case, exeptions being Wormtail and Snape.

Also, we need to keep in mind that the majority of the story takes place during a war. In war, the individual is often not thought about, people favoring to take a whole country (for example) as one group with the same ideas. Since the main fighters on one side of the war were Slytherins, it makes sense for the people on the other side to just lump all Slytherins in together as one. Instead of individuals with different thoughts.