Tunisia and Egypt have fallen, Libya is tottering, Jordan, Algeria, and Bahrain are shaking, Iraq is simmering, the Saudis and Moroccans are shivering, Lebanon is Lebanon, Iran and Syria are smiling (nervously), and Western leaders are scratching their heads, unsure what is next. But who will be toppled, shaken, or just stirred, is not pot luck. By examining the nature of the regimes, we can better predict the leaders' fate. Simply put, it's the difference between dictators and despots.
Four factors are central to determining whether a protest movement of the sort seen in Egypt and Tunisia will overthrow a leader: the structure of the regime (is it purely authoritarian without even the pretense of democracy?); the army (is it strong and loyal to the leader?); the general population (do they fear the regime and believe its rhetoric?); and influential groups (in the Middle East, for example, do powerful tribes support the regime?). Despots come out well on all four factors, while dictators (however ruthless they are) fail to satisfy them.
Consider Egypt. While Mubarak was certainly authoritarian, he claimed Egypt was a parliamentary democracy and went through the pretense of elections. So when the protests came, his response was hampered by the façade he had built. Mubarak also crucially failed to ensure the loyalty of Egypt's powerful military. Its leaders had grown unhappy with the encroachment of Mubarak's son Gamal into areas of power and the economy that they had previously dominated. So when the protesters came, the army was happy to help usher Mubarak out.