The Roman Republic had several items in place to keep any one part of the government from having all the power, including a group of men called tribunes who could veto select items and completely stop acts of administration, limited terms and specific requirements for members in order to move up in the government. Many governments in the modern world are based, at least in part, on the model of the Roman Republic, including the United States.
The magistrates were the official ruling group, but in many ways, the Senate held even more power. The power of the Senate came from its control over the country's public finances, power over foreign affairs and even approved legislation that would be sent to the assemblies to be finalized.
The government was also particular at first about who could be citizens of Rome, but as time went on, more and more people were allowed to be citizens. This was a benefit when Rome conquered other countries. The newly conquered people were less likely to rebel if they were brought into Roman citizenship and given all the rights of the people around them.
Rome also took equality under the law very seriously. They had the 12 most important laws carved into tablets; while they were often considered harsh, they guaranteed equal treatment under the law.