Here is a heart warming post from The Oil Drum. It analyzes the issues which most likely lead to the downfall of the Roman Empire. The post linked to below is an abridged version; as the original was over 10,000 words long. Here's an exceptionally abridged version though:
The costs of almost every resource in their society simply became too high. Too high for a growing population. This is specifically seen in resources consumed, capital expenditures required, and agricultural demands. A growing cost on their society paralleled the growing complexity of their social structure.
So what can we take away from this? Presumably a correlation to todays ever complex societies and the demands we place on our resources. I have always considered another facet which may have contributed to the downfall of the Roman Empire. That is, the degree to which the masses came into power. This is to say that at some point the elite ruling class became obsolete, and the masses demanded their due. Think of this as a form of exploding entitlement demands. At some point, the people have demanded so much in the form of care from the government that the entire system breaks down.