Going off of Prof. Xavier Dalen's comments, I think it's the combination of our ambition and drive and the fact that we're willing to crawl out of whatever hardship we find ourselves in to achieve even GREATER things... I was discussing this with Brighid Katz (a Hufflepuff) and she remarked that one of her friends elsewhere said in analyzing Hogwarts houses that Slytherin struck her as a house that highly values self-preservation and that most people in general don't seem to like self-preservation as a trait, after all it's a trait cockroaches, pigeons and rats and other pests have in common -- thriving in what normal people consider crazy situations. I think it's a fitting description for Slytherin.