A corollary to this requirement is that the nation's "elites" must also be wise and benign since they will often be the ones who lead the nation. Just as Plato concluded 2000 years ago, the elites must place the interests of the people and state first, or you slide down a path of tyranny and social unrest. Mexico is a good example of a state where the elites (descendants of an exploitive colonial class) are out for their own interests at the expense of the "swinish masses," to quote Jefferson. This leads to revolution and economic chaos which is becoming apparent in the poor state of Chiapas.
Next , a large democracy must have a common cultural background. A shared heritage is imperative for a large democracy because it keeps cohesion to democracy's naturally disordered state. Without a unified set of goals and values a democracy can splinter, as America did in the Civil War. De Tocqueville noted America's strong grass roots where he commented on our predilection for forming associations that were not conscious of class in comparison to European society. A democracy may be pluralistic with respect to its acceptance of many religions and races but it must have a shared tradition. America does. It is found in the Constitution and civilian government, for example.
A large democracy must also have a free press to thrive. The suppression of free thought is a major step toward tyranny. Without an independent and watchful