This is a great question. I would say that the biggest consequence of World War I was that it lead to World War II, one of the most destructive wars in the history of the world.
The Germans thought that the treaty of Versailles (the end of World War I) was too harsh. For most the 1920s the German people were very poor and bitter. This bitterness lead to World War II and made possible the leadership of Hitler. When Hitler became chancellor, he started to build up his army secretly. The rest of the Europeans were not blind to this, but allowed it, because they also fear the growth of communism.
Second, In the aftermath of the World War I, the League of Nations was formed and failed to do its job. In theory it was great. A league that would mediate the conflict of nations would be a great thing to maintain peace, but it did nothing. Part of the reason for this was because economic depression hit many nations. For example, Japan attacked Manchuria to get more resources in view of their depression. The league did nothing. It seems as though there was a free for all.
Third, with Germany getting more agressive, other European nations used a stragtegy of appeasement. In essence they looked away. This was obviously the wrong move. Germany continue to be an agressive country to the point where World War II was started.
In short, the world did not learn from World War I. Hence a few decades later, there was another war!