What has been the greatest motivator for change throughout history - political discontent, religious passion, intellectual curiosity, geographic circumstance or economic prosperity?
Economic prosperity - or rather, the LACK of prosperity - is a key motivator.
1. In the late 1700s, climate became cooler which reduced grain harvests in Europe. In France, this led to laws that tried to control prices called "the law of the maximum." Faced with inflation, the French tried to limit the price of many items. A key item was bread, but when they placed a maximum price on bread, but the price didn't allow bakers to recover their costs, so they made less bread. This led to food riots, and a man named Ropespierre and a group called the Jacobins used those circumstances to fuel revolution. Today, we call that the French Revolution. See, for example, http://mises.org/daily/1504
2. In North Africa over the last several years, a combination of unemployment and poor food harvests have led to uprisingings in Tunisa, Libya, Egypt and other countries. Various Islamic groups have used the need for food and jobs to promote a revolution and creating new governments that apply a strickter interpretaion of Islamic law. See, for example, this article: Are Food Prices Approaching a Violent Tipping Point?
"A provocative new study suggests the timing of the Arab uprisings is linked to global food price spikes, and that prices will soon permanently be above the level which sparks conflicts" at http://www.guardian.co.uk/environment/damian-carrington-blog/2011/aug/25/food-price-arab-middle-east-protests
3. While there were many causes of the Crusades, unemployment and famine also played a role. From http://www.the-orb.net/textbooks/nelson/first_crusade.html , we have this quote:
"The economic system was in a state of transition, with some districts specializing in some "industrial" crops to the point that they did not raise enough grain to feed themselves, and were doing so before the transportation and internal trading system had advanced enough to distribute consumer goods efficiently. So there were frequent local famines. At the same time, agriculture was improving so greatly in productivity that many people no longer had work. The peasants needed more food and more land to cultivate. In 1095, a famine and epidemic in northern France and the Lowlands was causing widespread misery and the lower classes were some miracle to deliver them."
In conclusion, there are always many factors. Certainly, political discountent, religious feelings, and other causes are present - as is true in each of the above three examples cited. However, in all three cases - and others that could be named as well, a "triggering event" occurs that tips the scale and makes the masses willing to accept the sort of change being proposed by an "inner circle" of those seeking to create change.