Nick is different for two reasons: his upbringing (i.e. his Midwestern roots) and the fact he had nothing "preying" on him as Gatsby and so many other rich people of Long Island did.
Nick has not been corrupted by growing up around old money (or even new money) in the Northeast. Instead, he begins the novel by reporting the advice he was given from his father:
Whenever you feel like criticizing any one ... just remember that all the people haven't had the advantages that you've had.
(Compare that with what is going on when we first meet Gatsby: a wild and reckless Roaring Twenties type of party.) This quote at least semi-confirms his good upbringing. True, James Gatz attempted a similar upbringing for Gatsby, but did not succeed. Obviously Nick's father made a true impression on his son. In fact, the only reason why he approached the East at all is that after returning from the war,
Instead of being the warm center of the world, the Middle West now seemed like the ragged edge of the universe--so I decided to go East and learn the bond business.
Big mistake, dear. Ha! You'll notice, of course, that by the end of the novel that Nick flees back to the Northwest to escape all of the despicable corruption he observed.
There is a quote from Nick in the very first few pages of the novel that highlights my second reason:
It is what preyed on Gatsby, what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive sorrows and shortwinded elations of men.
Gatsby's obsession with Daisy preyed upon him. Gatsby was, therefore, corrupted as the rest of the society was. Daisy's shallow upbringing preyed upon her and created a shallow person searching desperately for security. Tom's upbringing in the midst of wealth only served to elevate his pride and arrogance in the midst of old money.
Even though Nick has a fleeting moment of infatuation with Jordan, Nick has been far too rooted in the sound, just values of the Midwest to have much "prey" upon him at all. Although many consider him to be an unreliable narrator, I simply think of him as a just man trying to make sense of it all and fleeing back to uncorrupted safety. If you note how the other "selfish" characters are "preyed upon" in the paragraph above, you will see just how different Nick handles the same situations.
Congrats, sweet little Nick, for avoiding corruption!