Nick’s description of himself in the opening chapter holds true throughout the novel: he is tolerant and slow to judge, someone with whom people feel comfortable sharing their secrets. His willingness to describe himself and the contours of his thoughts even when they are inconsistent or incomplete—his conflicted feelings about Gatsby, makes him seem trustworthy and thoughtful.
Nick's father told him to remember before you judge others that they have not the same advantages as you have. Some lack the "fundamental decencies" that others had. And it is because of this belief, that Nick reserves his judgements about people. He does not reveal his true opinions of people and thus creates within them a false sense of trust; people feel comfortable telling Nick their secrets because he does not seem to judge.
Because of this, what he thinks of a person and how he interacts with the person are seemingly separate and contradictory. It is this fact that makes him an unreliable narrator. I think that this notion is the most poignant in his last meeting with Wolfshiem. To the reader, Nick makes Wolfshiem out to be the worse friend. Woflshiem refuses to go to Gatsby's funeral, in which Nick has set up. However, it was Wollfshiem who had been with Gatsby's in those first years, helping him. We thus have to question Nick's "friendship" with Gatsby. Was he ever a true friend at all? Or rather, was he a better friend than Wolfshiem? Nick leaves Gatsby pining away in front of Daisy's home after Myrtle's death just after witnessing Daisy and Tom making up. Not once does he tell Gatsby the reality of his dream - Daisy would not leave Tom to be with him. And his refusal to do so all stems back to his father's advice. Gatby was not rewarded the fundamental decencies afforded to Tom and Daisy, and so had not wished to crush this dream of his. Whether or not Nick was right in not telling him or not is entirely up to the reader. But as a friend, in my opinion, Nick had an obligation to Gatsby to tell him.
Not to mention that Gatsby was everything that Nick scorned... but my memory of the book wanes and to complete this response would cost me too much effort. I guess its time for me to reread the book~