The best way to sum up the difference between World War I and previous wars is to say that WWI was a “total war.” This made it different from previous wars militarily, politically, and economically. Other wars had been showing that the world was moving toward a war like WWI, but no previous war was truly the same.
Militarily, WWI was the war that depended most on mass armies with high technology fighting for long periods of time. WWI is famous for its new technologies. It is the war that introduced mass use of machine guns. It introduced tanks and aircraft. It introduced the use of poison gas. The amount of technology available made it qualitatively different from previous wars, making it practically impossible for major attacks to succeed once the war bogged down into trench warfare. WWI had huge national armies made up of millions of men. This was not typical of previous wars. WWI was a war that continued non-stop for years. Previous wars had typically been more like series of campaigns. This sort of mass war in which the fighting did not stop was new.
Much of this was due to economic changes. This was the first truly industrial war. The major nations of the world had become heavily industrialized by 1914. This allowed them to churn out huge amounts of war materiel. Industrialization allowed them to do this even though they were missing huge chunks of their labor force. Previous wars had been fought be less developed economies that had less productive capacity.
The military differences were also due to political differences. This was the first war of a deeply patriotic age. People still had faith in their governments and faith in the development of humanity. They felt that they could win the war and that the sacrifices were worthwhile. They believed that, by winning, they could improve the world. These sorts of attitudes helped make it possible for countries to sustain the sort of total effort that was needed. Practically everyone had to be willing to play a role. Young men needed to be willing to go and fight. Women, along with older men, had to be willing to work in roles they might not have previously held. There had to be a sense that they were all in it together and all needed to do their parts.
These factors combined to make WWI qualitatively different from previous wars.