I think another impact has to be the impact on the nations which were destroyed by the fighting taking place in their countries. The cost of rebuilding was enormous. Also as mentioned above it did set in motion the Germans desire for vengeance and the beginnings of WWII.
That's a very broad question depending on what you include in the definition of impact, but let me get you started on what I believe to be a few important impacts.
Human Cost - 16 million people, both military and civilian, died in this war. Entire generations of young men almost wiped away. In 1919, the year after the war was over in France, there were 15 women for every man between the ages of 18 and 30. All of the lost potential, all of the writers, artists, teachers, inventors and leaders that were killed were permanently denied to the future of the nations.
Imperial Realignment - The Austro-Hungarian Empire was broken up and drastically shrunk, while Poland, Czechoslovakia and Yugoslavia were all born as new nations. The nation of Germany was also smaller, and the Ottoman's of Turkey were finished. The Kaiser went into exile, and Germany plunged into economic and political chaos.
Pacifism/Despair - The postwar generation in Europe really believed it had been the "War to End All Wars" in that they never wanted to experience such a catastrophe again, and for a short time during the 1920s, a wave of pacifist literature and art fluorished in France and Germany. Others, damaged by the war physically or emotionally, or having lost loved ones, had lost faith in humanity given the terrible destruction the war brought.
In my opinion, the major impact of the Great War is that it set in motion the events that would lead to WWII. Since WWII changed the face of Europe and the world, I think that this is really a tremendous impact.
The Great War led to WWII because it made Germany into an angry and embittered country that had many problems. It also caused Italy to feel unhappy. These two countries' dissatisfaction was what led to WWII.
The Treaty of Versailles did things such as taking away German territory and imposing harsh reparations on Germany. This made Germans very bitter and caused them to want revenge.
The Great War led to the Treaty of Versailles and that led to WWII. That is, to me, the most important impact of the war.
Europe wouldn't be what it is today if it was the war. It was change in a way everybody will remember and know.
Like any war it is bad, but it happened. There were destruction of whole cities, people died. But after that there were improvements in the medicine for example.
There were big improvements in the technologies, also.
Months of peacemaking, actually years for some parts of Europe, involved time lags in setting up new machinery like the League of Nations; a prolonged hiatus requiring armies of occupation and continuing military service. Dangerously slow, then accelerated processes of demobilisation. Canadian and other armies riot; widespread fear of the veterans, and the role of veteran organisations in 1920s European politics; over time the veterans become a more conservative force. Though the fighting ended in November 1918, peace was not formally made until the Treaty of Versailles was signed in June 1919; it took even longer to return the countries involved to a true state of peace. The 1920s were though a“post-war” world in the sense that almost everything was conditioned by memories of the fighting and the disruption of war. Thousands of memorials were erected to war heroes and war’s casualties, and large numbers of celebratory books were published. In 1927-30, though, a more critical view of the Great War began to appear in books and films, a mood of disillusion that was naturally reinforced as another world war approached. As the 1960s and 1970s showed, each generation has reinvented the Great War to suit its own core beliefs, and it remains a topic both of great interest and of lively dispute. Disillusion and protest in the 1930s and the 1960s. Germans take to the streets in Berlin after learning the harsh terms for peace. Veteran radicalism continued though on pensions issues; the unemployed hero, demands for educational and land reform; France’s mutilés de guerreto be seen even as late as the 1960s. Continuing international disputes drawn from the war, occasioned by Reparations right up to the 1929 Wall Street crash. The desire to return to normal is endlessly delayed as there is no proper moment of closure. No end to “the war to end wars” and creeping disappointment as a result. A tide of routine and institutional history books appears in the 1920s, still illustrating a good market for an upbeat view of the war.