One slightly annoying difference between William Golding's novel, Lord of the Flies, and the 1990 movie adaptation is that the characters swear--a lot. Though that might be a more realistic portrayal of a group of young boys left unsupervised on an island, it is not true to the book. This meaningless foul language detracts from the greater misdeeds that the boys commit while on the island.
Two other differences between the novel and the movie are much larger and have a much greater impact on the story. First, the boys are American, not English. This is another way in which the impact of the boys' deterioration into savagery is minimized. While American boys should certainly not be characterized as savages, they are not as "civilized" as a group of uniformed English schoolboys fresh from their strict boarding schools. The decline from American schoolboys to savages is not as steep as the decline from proper English schoolboys to savages--and this is the primary theme of Golding's novel. Making the boys American dilutes this.
The second difference is even more retrograde to Golding's theme that, without the restraints of civility and authority, human nature naturally devolves into savagery. He demonstrates this effectively in the novel because he puts no adults on the island. In the movie, however, the pilot of the plane on which the boys were riding survives the crash. Once again, this change conflicts with Golding's theme because there is an authority figure on the island, even if all the boys do not know he is there.
These two changes may make the movie more popular for an American audience, but they detract from the powerful impact of the novel's themes.