The second advantage of democracy: rights. What enables the people to control their governments, is that they have rights. Under democratic constitutions, citizens have the right to life, the right to speak, the right of assembly, the right to discuss, the right to information, the right to criticise, the right to worship (or not to worship), the right to publish, the right to property, the right to fair trial, the right to vote. Furthermore, democratic constitutions impose on the state a duty to respect the rights of citizens and to maintain institutions dedicated to their protection, such as an independent judiciary. If a constitution does not enshrine basic rights and ensure institutions for their maintenance, it is not democratic. Under autocracy, governments have rights, not people, or at least power from above trumps rights from below.
For more detailed analysis, see How Democracies Live.
On the absence of rights under autocracy, see The Truth About China.