The opening paragraphs, in addition to being exquisitly written, help to establish for the reader two main things: Nick's narrative style and an aura of mystery about Gatsby.  We can learn from Nick's "voice" that he is an educated person with a solid family background.  His sophisticated vocabulary and his manner of speech clearly indicate a person of good breeding.  The lengthy description of his family history gives him a sense of history and authority.  If you read this section carefully, you will notice that Nick's family history parallels the development of America.  They came over from Europe, established themselves in the East and than migrated to the Midwest.  The fact that he uses the word "snobbishly" twice is also a clue to where his family sees itself.  The point is that we, as readers, clearly understand Nick's background.  We do not, however, have much to go on when it comes to Gatsby.

The second major point of these initial paragraphs is what we learn about Gatsby.  We understand, first and foremost, that Nick is conflicted about his own judgment of Gatsby (and someone who says he "reserves judgment" at that).  Gatsby is someone who represents "everything for which [he has] an unaffected scorn," yet "there was something gorgeous about him."  We also learn that Nick admires Gatsby's "gift for hope" and "romantic readiness," and that he "turned out all right in the end."  I urge my students not to forget these main characteristics of Gatsby, because it is this, plus what Nick says about him in Chapter 8 ("They're a rotten crowd.  You're worth the whole damn bunch put together) that make Gatsby great. 

Armed with this knowledge, readers can begin to see how Nick will become our guide through the sordid events of summer 1922 in New York, and Jay Gatsby will be our misguided romantic hero. 

Irony is present in the opening paragraphs of The Great Gatsby.  Things are not as they seem.  Nick presents the illusion that he is unbiased and nonjudgmental.  This is what is set up in the opening paragraphs.  The novel is full of illusion, and blindness is everywhere. 

Nick serves up an anecdote describing how his father taught him not to judge others, because others might not have had the advantages that he had.  He says he is "inclined to reserve all judgments." 

Then he serves up judgments, describing others as "bores" and "abnormal." 

Nick is deceiving himself--holding on to illusion.

Even the anecdote demonstrates this--you don't have to concentrate on not criticizing others, if you don't think you are better than others. 

Illusion is set up in the opening paragraphs of the novel.  This leads into Gatsby's illusion, Wilson's blindness to his wife's affair, Owl Eyes, the poster for an eye doctor overlooking the Valley of Ashes, etc.    

I think Nick using these paragraphs to set up his reputation with the reader from the get-go. He wants the reader to trust him, and thus gives example and cause for us to do so.

In the twenties, there must have been the idea that people regularly lied. Nick worked hard not to just say that he was honest or that he didn't lie, he built description and situations into that. Nick wanted us not to second guess what the narrator said. This is really important to catch because often when people tell us they are not lying, they actually are.

In many ways, I would argue that they serve to set up the main conflict of the novel, the idea of wealth being something that can not just lift someone up, but will also likely corrupt in a way that very few people seem to be able to resist.  It also sets up Nick as the narrator and in some ways sets him apart as someone who does not tell the story to criticize other people but just to tell the story as an observer.  Of course this isn't completely the case, but it serves to set that up regardless.

He also makes it clear that he is in many ways the guy that other folks talk to.  He is the person that people seem able to trust and so they confide in him.  Again this is helping to set up his legitimacy as the narrator.