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.

He believes that he's the most honest person he had ever known in his life(not narcissist or anything).