No one is completely non-judgmental, not even in literature. While Nick tells us, in the opening of the book, that because his father once told him to remember that he's had advantages in life others haven't had, he tends to reserve judgment, that doesn't mean he never passes judgment on a person - he just doesn't make immediate evaluations that remain unchanged. In fact, it's only one paragraph later, that Nick reveals that he didn't care much for Gatsby at first. He says that Gatsby represented everything that Nick scorned, but that Gatsby turned out to be an OK guy, in fact, he turned out to be a guy that Nick admired. Nick's last words to Jay Gatsby, in chapter 8, were, "They are a rotten crowd. You're worth the whole damn bunch put together." What makes Nick a valuable narrator though and what his point is in the opening of the story, is that Nick doesn't make snap judgments on people for the most part. Nick tends to be open to people and he judges them finally by their behaviors and their attitudes revealed to him over time. After having spent an evening with the Buchanans in the first chapter, we are told that he left feeling a little disgusted. His disgust dealt with the lack of morality he witnessed, namely, Tom's affair with Myrtle and the fact that Daisy was obviously aware of it and did nothing about it. His opinion didn't stop him from giving the Buchanans opportunities to change that opinion. Just because he didn't agree with their moral choices, he didn't shun them. This all points toward his lack of being a judgmental person and narrator. His value as a narrator is that he is fairly honest in telling us, the reader, what is happening and letting us form our own judgments. We can trust him because he doesn't insert a great deal of opinion.
If you look at the fact that Nick is our narrator and is a character in the story the only image we can make of other characters is based off souly what he does or does not tell us about them. because he is apart of the story he cannot entirly pass a non-judgmental veiw of them. he can easily be swayed by emotional connections to the characters, as demonstrated with his veiws and ties to Gatsby. he may state in chapter one that he reserves judgment but does he really? that is the question to really ask. we dont know the story from any other veiw but his own. and being a narrator and character he can shape the characters what ever way he may. he, to me, has a very bias opposition when it comes mainly to how we see other charaters and Gatsby.