Fitzgerald uses the metaphor of the "green light" to signify the hoped-for future that we continue to believe is a possibility but that actually gets further and further away from us.  We think to ourselves, we didn't reach it yesterday, but tomorrow we will be sure to try harder, and so we convince ourselves that we cannot fail to achieve success.  We continue to believe that "one fine morning," our dreams will, in fact, be within our grasp and not somewhere in that distant future anymore.  However, despite our belief, we are actually pushed backward, further from our dream the more we try to achieve it.  Another metaphor compares us to boats that are "ceaselessly" pushed back by the current, a metaphor for reality that would prevent our hopes from coming true, for the failure of the American Dream.  

The level of diction here is standard: it exists above conversational/neutral (our everyday speech) and below elevated (language that is often considered sacred and so is rarely changed).  It is the kind of language that we often use in formal writing and the like.  Words such as orgiastic (instead of emotional or even frenzied, for example), eluded (instead of escaped or outran), and ceaselessly (instead of never stopping) help to indicate the diction level.  

Tone, in literature, refers to the author's feelings about the subject.  In this case, it does seem as though the author is in agreement with Nick Carraway, the narrator, as we have seen this description play out throughout the text.  Therefore, we might describe the tone as knowing or cynical.  

This passage essentially describes what Carraway (and, likely, Fitzgerald) believe to be the human condition, at least for Americans living during the 1920s.  There's this glitz and glamour about the age that comes from the clothing, the music, the dancing and entertainment industry.  However, there's a tragedy about it too: the idea that the American Dream is a fiction that everyone wanted to believe in but that people really were not able to achieve.  It was, simply, a "dream" in a true sense of the word.  And there is something quite tragic, if naively innocent, about people's commitment to their belief in the possibility that this dream could be made reality when, in reality, we cannot reach it.