There are numerous aspects of our modern society that are outlawed in the futuristic setting of Fahrenheit 451.
Most notably, possessing books and reading books are against the law in Montag's society. Firemen are no longer the protectors of society as we see them today; instead, they are charged with keeping society "safe" by burning books wherever they are found. This discourages independent thinking, which is a crucial aspect of maintaining a populace of shallow thinkers who are compliant with the rules of their government. Therefore, by extension, freedom of thought and intellectualism are also effectively illegal.
It's clear that Montag's society has many rules to govern behavior. In fact, Clarisse tells him that her "uncle was arrested ... for being a pedestrian." People are expected to drive fast; travelling at a mere forty miles an hour can be justifiable cause for serving two days in jail. If the government arrests people for driving too slowly, it can be inferred that they have a heavy hand in governing other aspects of citizens' lives.
Due process of law has also been eliminated. When books are found, there is no real investigation and no sense of defense for the offenders. Instead, those who break the rules are subjected to having their houses burned down immediately and may even be killed.