Nick's description of Tom in Chapter 1 demonstrates a couple of truths about Nick and Tom. First, at the beginning of Chapter 1, Nick describes himself as an objective, good listener. He believes that he is the type of person who does not make judgments quickly about other people. However, after Nick describes his own family background and his reason for moving East, he lapses into a description of Tom Buchanan. Ironically, his tone is hardly objective when it comes to Tom. Instead, Nick's use of phrases such as "cruel body" and "hard mouth" illustrate his view of Tom as a bullish, intimidating figure. In summary, his tone (or attitude) regarding Tom is critical and slightly censorious.
Fitzgerald, of course, includes Nick's description of Tom first to foreshadow not only events near the book's end but also to hint at Daisy's strange, ditsy behavior during Chapter One's dinner. By introducing Tom as an excessively wealthy bully, the author sets the scene for Daisy's disillusionment with her marriage to Tom (she makes awkward comments at the dinner table when Tom's mistress calls) and with her lot (and her daughter's) in life as a female.