Patrick S. answered 08/26/22
History Tutor and College Admissions Consulting
In addition to the end of monarchies (Russian, Austro-Hungarian, Ottoman, and Germany) and setting the stage for WW2, you can argue that WW1 was a turning point in the way wars were fought, the types of ideologies that emerged, and change in the geopolitical order of the time.
The First World War was one of the first truly industrialized global war. The number of casualties and the capability of states fielding millions of soldiers is a testament to the industrialization of war. Do not forget that in addition to European states, thousands if not millions of troops were also drafted from European colonies in Asia and Africa. This is particularly important when looking at the global Decolonization Movement that emerges, especially in India when joining the War on the side of the British was an attempt by the leaders of the subcontinent to eventually transition to self-government.
In addition Decolonization, the collapse of monarchies and the economic devastation that followed gave rise to ideologies such as Communism and Fascism. The October Revolution, the Rise of Hitler, and fascist government in Japan also point to WW1 being a turning point.
Shifting geopolitics and the global order of the time can be argued to be one of the most important results of the First World War. The war saw the waning of European power and the rise of the United States and Japan. Both benefited from the First World War. The Japanese economy took off as exports rose dramatically due to lack of production in Europe while we see America flexing its economic muscles as the world's largest economy and slowly replacing London as the financial center of the world.
The First World War can also be seen as an end to the European scramble for colonies. By the eve of the war, most of the globe had fallen under European dominance whether directly or indirectly. You can argue that since Discovery of the America's, European states have been racing for colonies (and the markets, resources, and labor that come with it) and that the First World War occured simply because there was no where else for rising powers like Germany to expand, hence triggering this final Continental European showdown.