The first main lesson of the novel is the importance of individualism versus collectivism. All the people who conform to society are ignorant and incapable of critical thought. Clarisse, who is an individualist, seems more able to observe and judge the strange society -- such as the legitimization of hit-and-run accidents as acceptable catharsis -- and so is deemed a threat to society.
Another lesson is the importance of differing opinions. Everyone in the future society of the novel thinks the same way, likes the same things, and has the same opinions. There is no difference in thought and so there is no reason to question what is shown as fact by the government. This causes people to think that the impending war is not a very important issue; the war later destroys the entire city.
Another lesson is the importance of content versus emotion. All the television programs are meaningless, not even telling a story, because all they do is appeal to emotion without any larger context. Chief Beatty explains:
"More sports for everyone, group spirit, fun, and you don't have to think, eh? Organize and organize and superorganize super-super sports. More cartoons in books. More pictures. The mind drinks less and less. Impatience."
(Bradbury, Fahrenheit 451, Google Books)
Effects like this can be seen in modern times, when a focus on fast information and fast entertainment has caused attention spans to shorten. Without focusing on real content with meaningful information, the mind gets distracted and confused.