Nick, the narrator of The Great Gatsby, goes to great lengths to tell us that he concentrates on not making quick judgments and not judging others. He also tells the reader how honest he is later on in the work.
The fact that Nick thinks these things about himself just proves that he suffers from the same thing everyone else in the book suffers from: believing in illusions. Nick is under the illusion that he doesn't make value judgments about people, when in actuality he does.
An astute reader catches on to this immediately, when Nick talks about how his father taught him long ago not to judge others harshly, because others have not had the advantages that he has. The fact that his father taught him this shows that his father thinks he is superior to other people, and that he thinks Nick is superior to other people. If one doesn't naturally think you're better than other people then you don't have to concentrate on not judging them harshly. A feeling of superiority comes naturally to Nick by the time he starts narrating the novel.
In short, Nick tends to see people from the viewpoint of the Midwest. He judges them according to his Midwestern values. Jordan is uppity and lazy the first time he meets her, for instance. That's his first impression of her, even though he says on page one that he doesn't make first impressions. She might appear to be mellow, to someone from California who is used to lounging on the beach. But to Nick she appears lazy.
But one shouldn't judge Nick too harshly. First of all, again, everyone else in the book falls for illusions, as well. Second, all people are egocentric. Nick is a character in a sophisticated novel. One wouldn't expect him to be perfect. The fact that he makes judgments is not unexpected, and neither is the fact that he has illusions about himself--who doesn't?