Fossil fuels have changed society because they have an extraordinary Energy Return on Energy Input (EREI. This can also be known as Energy Return on Energy Investment). This has meant that in the early days it was possible to expend the energy of one barrel of oil and get something like 100 barrels back. Lots of calculations have been made to show how many hours of human type labour will be generated by a single petrol tank. Coal, oil and gas have increased the amount of energy available to humans almost immeasurably. They are easy to process, easy to harness, easy to burn and (through simple technology) easy to direct and focus. What we can call the ‘social relations of energy’ have made the modern world possible, and been influenced (and reinforced) in turn by that modern world.
The main drawback of fossil fuels is pollution in the getting, transport and burning, and the consequences of that pollution, which includes bad health and climate change. There is some evidence to suggest that it is getting harder to find oil, as people are seriously considering tar sands, which have a very poor EREI, and deep sea oil, which is difficult and dangerous. There is still plenty of coal and gas, as far as we know, certainly more than enough to cause runaway climate change, but eventually they will likely run out as well. However, it could be suggested that if it were not for the pollution from fossil fuels than humans would not be attempting to abandon them.
EREI is hard to measure precisely, as technologies exist in complex social and ecological systems, everything links to everything else, and it is often hard to draw firm boundaries around a technology, supply chains and energy usage in those chains. However, The more available and directable energy which arises, per unit of energy used to activate that energy, then the more easily-available energy there is and the more freedom of action a society has.