What quality does Nick possess that he thinks makes him different in The Great Gatsby?
Nick was told by his father, at some point before the time of the adventures retold in The Great Gatsby, that "a sense of the fundamental decencies is parcelled out unequally at birth" and that he, Nick, should remember that he is advantaged over many others in this regard.
Nick accepts this statement from his father as being true in his case, and sees it as being an important influence in shaping the way in which he interacts with other people. In particular, Nick considers himself more open-minded and less judgmental than most other people. This is a mixed blessing at times, as he comments,
I'm inclined to reserve all judgments, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores.
Overall, however, Nick considers his openness to whatever personality quirks he observes in others as being a unique asset in his personality.