Hemingway tells us a story here about the fragility of relationships, the impotence of generosity in certain situations, and the nature of time as moving always forward, despite our deepest desires that at time it can be turned back.
I think what strikes me most about this story is how language operates on so many levels. To understand this story truly you have to be able to read between the lines and consider what is being not said just as much as what is being said. What is fascinating about this story is the sheer amount of dialogue that is just reported with little or no comment from the narrator - we become eavesdroppers trying to piece together the tragedy that is about to happen.
As this story was written in 1926 between the two World Wars and after Hemingway had been involved in war efforts as an ambulance driver, his experiences continued to influence his thought. The terrible sense of man's alienation and isolation that Hemingway felt permeates this story. His male character captures this sense of aimlessness and loss of values that Hemingway felt was characteristic of the "lost generation."
First, Hemingway experiments with the limitation of language. Through the couples' strained dialogue, "Hills..." explores the painful inadequacy of communication. The story's central tension creates a great place for Hemingway to develop and test his terse, limited, and concise prose style. If you leave the story confused, then Hemingway has succeeds in pointing out that language itself is often broken, powerless, and frustrating.
Second, the story explores the dark side of the carefree, drunken, and exotic life of American expatriates in the 1920s -- a life that Hemingway lived and knew well. Without resorting to the heavy-handed moralizing found in much of early-twentieth-century American literature, Hemingway's story reminds his readers that the every good party is followed by a bad hangover. In part, the story is great because it delivers a strong, old-fashioned, Victorian, moral warning in a style that is wonderfully indirect, intriguing, engaging, and modernist. In other words, Hemingway gets away with a bit of preaching about the cost of sexual freedom without ever sounding 'preachy'.
Third, Hemingway offers a feminist message. Yes, Hemingway. Yes, feminist. By the end of the story, Hemingway succeeds in making the man, and to some extent men, seem cowardly, selfish, and detestable. At the same time, the woman earns a degree of our pity when we realize what she must put up with. However, the woman is not pitifully weak. Her now-famous request that the man "please, please, please... stop talking" is a moment of empowerment in a story that otherwise offers only disempowered language. Because the man is pushy, deceptive, and manipulative about the abortion; and because the woman sees the man's weakness and worthlessness; the story offers a warning to women. The story suggests that women watch out for themselves, rely on themselves, and avoid the sexual traps that men set. By making us feel just how horribly trapped the woman is, Hemingway asks women: would you want to be in her situation? If not, watch out for manipulative men and rely on your own female judgement and decisions.