Following this question I asked myself why the Sorting Hat would see Slytherin as a good option for Harry.
As far as I know Harry wasn't cunning, he wasn't pureblood, nor was he power hungry or had great ambition.
But still the Sorting Hat said that Slytherin could lead him to greatness even if he had none of their values.
"Not Slytherin, eh?" said the small voice. "Are you sure? You could be great, you know, it's all here in your head, and Slytherin will help you on the way to greatness, no doubt about that -- no? Well, if you're sure -- better be GRYFFINDOR!