What quality does nick possess that he thinks makes him different?

Nick does not come from the rich establishment. He comes from upper middle class. He fought in WW1 and feels pretty grounded. He seems contented with not having much and living in West Egg. Nick differentiates himself from the ultra rich characters in East Egg. He finds them superficial and trite. He even separates himself from Gatsby who he is intrigued by but also skeptical of. While Nick is not the perfect narrator, he is reliable. Nick certainly stands out as more grounded than any other of the characters in the story.