This is a great question. There is a recent book out that actually deals with this very topic. The author is David Graebel and the book is called:The History of Debt: the First Five Thousand Years. This is an excellent book. I will add the blog. The author argues that revolutions take place chiefly on account of debt. When people are no longer able to pay their debts, there is a reshuffling that take place.
If you think about this thesis, it holds a lot of water these days. Think of what is happening in the Middle East. Much of this is due to rise of food prices, inflation and debt. Think about the Occupy Wall Street rallies around the world. The point is about debt and greed. If you think about the French Revolution, money was also a central issue. Finally, as we think about the problems in the West, much of it is about debt. It is all over the news. In light of this, I would say that revolutions take place mainly over debt.