The biggest change was the move from Axial-flow turbo jet engines, like the Rolls Royce Avon, to more fuel efficient and quieter turbo-fan engines, like the General Electric CF-6. Airlines started to try anything to save weight and many stopped painting the fuselages of their aircraft. The very earliest jet engines used on commercial airliners, like the Boeing 707, burned so much fuel, they could not fly from New York City to Los Angeles, without making an intermediate stop. Flights from New York City to western Europe had to stop in Gander Newfoundland to refuel.
The Rolls Royce Avon jet engine was every reliable. More than 11,000 were built. They powered most of the European built commercial jetliners and Lockheed used the Avon on their L-1011 wide body jets. However, the post 1973 oil shock led airlines around the world to turn to the more fuel efficient turbo-fan jet engines. Turbo-fans have an additional benefit. They are much more quieter than the early axial flow turbo jets.
Source: "Classic American Airlines" by Geza Szurovy (2000)