When the framers of the Constitution set out to lay the groundwork for the U.S. criminal justice system they were extremely concerned about tyranny. Specifically, they were concerned about the way that criminal justice systems had historically been used by tyrants to persecute their political enemies. They envisioned a justice system that would make such persecution very difficult. Although no criminal justice system is perfect, the U.S. criminal justice system has a number of strengths to its design.
Presumption of Innocence
Under the U.S. system of justice individuals are innocent until proven guilty. It is also unconstitutional to deprive an individual of "life, liberty, or property, without due process of law." Someone cannot be convicted of a criminal offense without the charges being proved "beyond a reasonable doubt," and a person cannot be tried for the same crime twice. This means that if any branch of U.S. government, including state and local government, arrests someone the burden is fully on the state to prove that the person has done something wrong and deserves to be punished.
The burden of finding evidence also falls on law enforcement. Various protections prevent the state from forcing a person to assist in his own prosecution. For example, under the 5th Amendment to the Constitution a person cannot be forced to testify against himself. He also is protected against "unreasonable search and seizure." This has been interpreted as offering a variety of protections. For example, police cannot search the person or property of an individual without being able to show probable cause to a judge and get a warrant. This has been extended to the right of the state to intercept phone calls or email.
The state has a tremendous amount of power and money. Precautions were taken to ensure that an individual could not easily be overwhelmed by that power. All defendants, for example, have the right to legal representation. The legal system can be complex, and individuals who aren't trained in law may not understand everything that is being discussed, the rights that they have or the full implications of what is happening to them. Defendants also have a nearly limited right to appeal. If, for any reason, their trial is deemed not to have been fair or if new information comes to light, convicted persons can try again to show that they are innocent.
Perhaps the greatest protection offered to individuals is people. Serious criminal charges require a jury, and that jury must be unanimous in determining guilt "beyond a reasonable doubt." The judges, too, have great discretion when it comes to trying and sentencing individuals. There are a few exceptions, such as minimum sentencing laws, but generally a judge can take the circumstances of a crime into account. The constitution and various state and federal laws set the rules for court proceedings and prosecutions, but ultimately a person must be convicted and sentenced by other people and not a set of pre-determined rules.