This is a good question, but difficult to answer satisfactorily because the idea of "good" is pretty subjective, and it's easy to combine "good" with "productive", and then we end up saying things like "improvements in machine gun technology was good". Basically we need to ask "good for whom?" in order to answer this question, since the good or bad depends entirely upon one's stakes in the war's outcome.
A few objectively good things were:
- The war made the need for a unified, cooperative international congress very clear. The fact that the League of Nations turned out to be weak and ineffective was more of a problem in its execution than in its concept. An international congress would make it much easier to legislate and negotiate over the kinds of complicated affairs that led to the war, without having to establish and re-establish the committee members and locations every time one was needed.
- The ban on chemical weapons was re-affirmed after the war and few nations developed significant stockpiles, as demonstrated by their absence during WW2.
- Research and understanding of anxiety and stress-related illnesses, such as "shell shock" became better recognized and understood, though they were fiercely coupled to Victorian notions of cowardice.
- A number of colonies and other imperial territories, such as Canada, gained a greater degree of independence and national identity due to the conduct of their servicemen in the war.
- A number of technologies with non-military applications, such as automobiles, airplanes, telephones and the Haber process became ubiquitous.
It is hard to imagine any war as "good", but World War I did produce some very positive aspects for the United States and the world. Two of those aspects included:
The formation of the League of Nations. The goal of the League of Nations was to prevent future war. The formation raised the idea of cooperation between Nations. Later this idea would be transformed to develop the United Nations.
Changing role of Women. World War I allowed women to take on new roles. Jobs Necessity allowed women to take on jobs previously closed to them. The men were at war so the women at home needed to work outside the home to support the war effort and keep the home fires burning. The war was important in giving women voting rights in 1919. The role of women changed forever because of World War I.