What were the most significant aspects of the civil and military systems of the Persian Empire? Were personal or were institutional elements more important in determining its efficiency and success? What evidence causes you to think as you do?
The Persian Empire was autocratic, capitalizing on a culture that respected (and still respects, largely) strength and order. As Post #2 points out though, there was still buy in from many local elements of the society not only because of their respect for authority, but because some limited forms of citizen participation and self government were allowed to exist--though still under the tight reins of the Emperor and his network of informers and police.
Iran today is much easier to understand if one has even a basic understanding of the Persian Empire and its history.
The most significant elements of the Persian Empire would be the Royal Persian Road and the efficient administration of the Empire. These were primarily civil institutions. The Royal Persion Road travelled 1600 miles, had a series of postal stations where couriers could obtain fresh horses, and was strictly policed to prevent robberies and other crimes. The road was so efficiently managed that couriers could travel its entire 1600 mile length in thirty days or less.
The Persian Emperor Darius instituted a remarkably efficient civil system in which he appointed governors known as Satraps to to rule within provinces of the Empire. Although the Satraps themselves were almost always Persian, lesser offices were filled with local people, thus giving them some voice in government, and hopefully less likely to rebel. To further guard against rebellion or insurrection, a series of secret police, known as the "eyes and ears of the king" travelled the Empire with supporting military. They often conducted surprise audits and inspections, and thereby learned quickly if rebellion were in the works. Through the efficient governmental operation, Darius was able to effectively rule an Empire that stretched over 1800 miles and encompassed over thirty million people.
Personal elements were much less important than institutional elements. Part of the success of the Persian Empire was its tolerance of local customs and religions practiced by the seventy ethnic groups within the Empire. When the Persian Emperor Xerxes attempted to change this and institute strictly Persian culture and language within the Empire, he provoked a series of wars which devolved into the Persian Wars. These wars ultimately prevented the further infiltration of Persian culture into the West, and assured that the culture of the West would be Greek/Western. It was his attacks on the Greek citizens of his Empire that Philip of Macedon used as an excuse to wage war against Persia. He of course did not live to prosecute the war; however his son Alexander (the Great) did, and effectively destroyed the Empire.