the Buddha and Confucius exist at a similar distance, where what they have inspired (as well as the myths and stories created around them) have eclipsed the historical reality of their lives.

In terms of similarities, I think earlier contributors were right when they stressed the importance of duties and obligations (I'd suggest that ethics, spoken more broadly, was key to all three traditions). Additionally, it's worth noting that life in moderation (while avoiding the excesses of asceticism on one side and self indulgence on the other) are key themes in all three traditions.

However, even if we hold that ethics are a key component to all traditions, distinctions can be made. Confucius, for example, was largely focused on social interactions and the obligations and duties which people have with each other. His was a very unequal vision which tended to distinguish between a superior and an inferior, with each side of that relationship having duties and obligations to the other. Plato, by contrast, was first and foremost interested in the cultivation of virtues. Plato was a moral realist, who held that morality and goodness were written into the universe, as part of a higher transcendent reality (this is most famously expressed with his Theory of the Forms). While relationships, proper behavior, obligations, and duties might be part of this, ultimately the true goal which superseded all others, as far as a Platonist would be concerned, is to cultivate virtue in oneself. The same would apply to Aristotle.

Additionally, it should be noted that these different philosophies attempted to address different questions. For example, Buddhism is ultimately grounded in the problem of suffering, which is perceived as endemic within the human condition. If there is a core question within Buddhism, that question is this: why do people suffer and how can we overcome it? Confucianism, on the other hand, is largely focused on questions of social stability, so the problem of suffering does not have quite the same primacy. The same applies to the Platonic and Aristotelian traditions, with tend to place a greater focus on individual excellence and self-actualization.

Buddha, like Plato, was interested in an understanding of the real world. Plato referred to it as the "escape from the cave," while Buddha called it "rising above the images of the wall."

Despite this need for knowledge of the outside world, Buddha, like Socrates was very interested in introspection.  Both philosophers believe that in order to proceed with an open mind, one must understand the limitations that exist due to one's ignorance. The pursuit of truth with an empty mind is essential to understanding oneself and the world around you. Buddha, like the other four philosophers, believed that knowledge was the true means of salvation, for oneself as well as the world at large.

Many of the beliefs of Buddha, like those of Socrates, specifically leaned towards humanism.  Buddha believed that everybody was responsible for the decisions they make and the outcomes of one's own life is not the result of intervention by gods or spirits.  Buddha, like Socrates, discouraged people from accepting religious truths and asked followers to use insightful reasoning and meditation to arrive at truth.  

While Buddha was not overly concerned with the political realm, he is on record, like the other philosophers, in believing that governments have a responsibility to lead by example and be just.  His belief that philosophical principles should guide and inspire leaders is very similar to the beliefs of Plato and Confucius.  Buddha was also very direct in his belief that greed leads to suffering, a point mentioned above.

The Greek philosophers and the great Chinese philosopher Confucius wanted to mentor leaders to create a perfect society.  Both groups stressed a censored, state education that taught virtues as well as political skills.  A wise ruling class was not possible except through careful education.  They stressed a need for philosopher kings that were perfectly trained to act as rulers.  Plato and Confucius both believed that if a king was not virtuous, he did not deserve sovereignty. None of the philosophers trusted the governed to rule and hence are interpreted as anti-democratic.  

All three of the philosophers taught that citizens had duties in this world and society could not function without people accepting these duties.  The Greek philosophers and Confucius agree that the purpose of the state is to serve the people.  The state should not harm individuals with taxes and too many laws.

There are three major differences between the views of Socrates and his students and that of Confucius.  Confucius believed that all men were conditioned to do good, while the Greek philosophers believed that most people acted selfishly or out of ignorance.  Another difference is the view of women.  Plato and Aristotle believed that if women were capable and talented, they had an important role in society to perform.  Confucius tends to favor a patriarchal relationship in which women are subordinate to men.  Confucius organizes society into relationships like father and son and the ruler and the ruled.  In the context of these relationships, he mentions the husband and wife relationship.  The husband acts as the ruler, and the wife, the governed.  The Greek philosophers were more interested in science and innovation, while Confucius felt that innovation would only be used for selfish gains.  

While Confucius wrote at a time and place different from those of Socrates, Plato, and Aristotle, much of his thoughts relate closely with the three Greek thinkers.  In his Analects, Confucius emphasizes the morality of the government and of individuals.  Everything derives from the efforts of individuals.  It was in their ability to perfect themselves, and in doing so would true change occur.  This change was directed as a return to the perfection of earlier times.  An extension of this is the idea of ancestor worship, an important aspect of Confucius's philosophy.  In addition, the pursuit of harmony was perceived as virtue in Confucianism. 

While these ideas may not seem very relatable to those of Socrates, Plato, and Aristotle, there are numerous threads connecting them.  Much of Socrates's thought is lost; the only real source of his philosophical outlook is through the words of his student, Plato.  Socrates, like Confucius, formulated a system built on social concerns, particularly those relating to justice.  From what can be gleaned from his thought, he spoke little, if at all, about natural concerns.  Like his Chinese predecessor, Socrates focused on human concerns.

Plato, rather than Socrates, offers a more elaborate and developed system of thought, and much of it can be related to Confucius's thought.  In The Republic, for example, Plato discusses many of the questions which concerned Confucius.  Like Confucius, Plato was concerned with the nature of government, particularly in terms of justice and morality, and he also posited that social change derives from the efforts of individuals to better themselves.  Like Confucius, Plato argues that an improvement in an individual's moral goodness will lead to an improvement in the morals of a society. 

Aristotle, unlike Confucius, Socrates, and Plato, had much more varied philosophical interests.  He writings cover not only human concerns but also the natural world.  He wrote works on ethics and politics, and a number of his ideas relate to those of Plato, Socrates, and Confucius.  In his Nichomachean Ethics, Aristotle argues that all human action is aimed toward a specific purpose, or the "good."  Unlike Confucius and Plato, Aristotle approaches the question of ethics rather scientifically, breaking down human nature into two impulses:  the rational and the nonrational. 

The rational, as the name indicates, deal with purely intellectual matters.   Morality, and its improvement, cannot be discussed or pursued unless the rational and the non-rational were included.  The non-rational comprises "vegetative functions," which essentially relates to those functions not associated with the intellect.  In Confucius, Socrates, and Plato, they place much less emphasis on the natural and more on the intellectual (an aspect of the human).  Like his three predecessors, however, Aristotle argues that the "good" is essentially a balance between two extremes.

Much of their similarities can be seen in specific works, particularly Confucius's Analects, Plato's Republic, and Aristotle's Nichomachean Ethics.