|Han Gaozu- Deified Founder of the Han Dynasty|
With that said, I find certain variables more interesting than others. Particularly fascinating is the impact that ideas and culture have on history. Partly because making judgments about other cultures runs against the ethos of multiculturalism that dominates today's ivory towers, and partly because claims about culture are difficult to quantify and prove, comparative analysis of cultural ideals and the way they shape history has fallen out of favor with many historians of world history. This modern aversion to placing culture squarely in the middle of history makes it all the more important to note those cases where ideas really made a difference.
These thoughts were prompted by a recent reading of Adrian Goldsworthy's How Rome Fell (Amazon.com review here). Goldsworthy's is a fine narrative history of the Roman Empire from the time of Marcus Aurelius to Justinian's failed attempt to retake Italy from the barbarians which had toppled the Western Empire. His is a good introduction to a complex topic - it is difficult to study the later Roman Empire and not be overwhelmed by the dizzying number of Emperors and would be Emperors that battled for the title of Caesar. From the third century onwards only a handful of Emperors died a natural death. The armies of the empire killed more Romans than barbarians; hardly a decade went by where one general or another did not declare himself to be Emperor. Some of these self declared Caesars died as they made their attempt for the throne; even more were killed by their own soldiers and subordinates after they donned the purple. Being emperor was a dangerous business.
The problem was foreseen by Tacitus almost a century before the true slaughter began. Describing the murder of the unpopular Emperor Nero, he wrote:
"Welcome as the death of Nero had been... it had also excited all the legions and their generals; for now had been divulged that secret of the empire, that emperors could be made elsewhere than Rome". 
It was a dangerous idea: any man could become Caesar, if only he commanded the armies to do so. If Goldsworthy is correct, and the constant civil war that racked Rome did as much to bring about its fall as the barbarians at her gates, then it was an idea that destroyed an empire.
The power that this idea (social scientists would probably call it a 'social norm') had on Roman history is all the more apparent when the Roman Empire is compared with another empire of antiquity: the Han Dynasty. What Rome is to the West the Han empire is to the Orient. The dynasty was a contemporary of Rome, lasting from 206 BC to 220 AD. Like the Romans, the Han controlled the world they knew. Unlike the Romans, the Han Empire was not afflicted with chronic civil war.
Why didn't the Han emperors fear their family and generals with the same obsessive paranoia as their Roman counterparts? I submit part of the answer lies with the Chinese conception of their emperor. Historian Mark Edward Lewis explains this ideology:
"The king of Qin claimed for himself the title of huangdi, which we inadequately translate as "emperor." Di had been the high god of the Shang, the first historical state in China that ruled the central Yellow River valley in the second half of the first millennium B.C. However, by the Warring States period its meaning had changed. The mythic culture- hero sages who had supposedly created human civilization were called di, indicating their superhuman power. And the four high gods of Qin religion were known as di, corresponding to the points of the compass and thus embodying the cosmos.
In claiming the title di for himself, the king of Qin asserted his godlike power, strengthened by the addition of huang, which meant "shining" or "splendid" and was most frequently used as an epithet of Heaven. Declaring himself the first huangdi, the First Emperor claimed t o he the initi- ator of a new era and, the progenitor of a second, third, and fourth huangdi, a dynasty that would reach to the end of time. just as his realm reached to the limits of space." The Romans also deified their Caesars. Yet when Roman rhetoric and history are viewed through the lens of their Han counterparts, you can't help but feel that they just didn't mean it. While he was happy to to adopt all sorts of religious imagery, the first Roman Emperor studiously tried to avoid the appearance of monarchy. The title he claimed for himself could not be more different its Chinese counterpart: He was the princeps, usually translated as "first citizen." It was not until the Tetrarchy, when the Senate was long banished to irrelevance and the Roman world had been rocked by a century of civil war, that Roman Emperors dropped the pretense that they were more than the first among equals. 
The Chinese situation was different. Decades before the China's Warring States were conquered and consolidated into one dynasty, Legalist philosophers and statesmen articulated the ideal of the centralized, autocratic state controlled by a mysterious and all powerful ruler. Confucians provided the justification for such a system, insisting that heaven itself would only allow ruling lines whose justness and humanity would bless the entire realm to come to power. If the dynasty lost these qualities, it would fall.
The idea of a "mandate from heaven" made armed insurrection against the Han dynasty a different game from that played by Roman generals. One did not fight to become Huangdi - one fought to overthrow an entire dynasty and everything that came with it. Chinese insurrectionists did not wage civil wars. They started revolutions.
These ideas left a powerful print on the statesmen who had them. Roman emperors believed that their personal survival depended upon how their soldiers perceived them. Chinese emperors believed that their dynasty's success depended upon how "the people" perceived it. Both acted accordingly.
 John Lewis Gaddis. The Landscape of History: How Historians Map the Past. (New York: Oxford University Press). 2002. Chapter 4. The entire book is a spirited defense of the historian's methods over those provided by social "sciences" that I would recommend to anybody.
 Tacitus 1.4 [link]
 Mark Edward Lewis. Early Chinese Empires: Qin and Han. (Harvard: Belknap Press). 2007. p. 52
 In pp. 157-159 of How Rome Fell: Death of a Superpower. (New Haven: Yale University Press) 2009. Mr. Godsworthy notes that even the artwork depicting emperors changed during the reigns of Diocletan and his fellow Tetrarchs.