Positive effects
The official symbol for NATO. (Click for larger view)
The creation of the league of nation, although proven to be a failure became the inspiration to world organizations that follow in world war II. The League of Nation's idea of collective security is still used by modern organizations such as NATO and the UN. The League of Nation also inspired for the establishment of global health organizations such as Red Cross. The league also helped solve social problems such as labor reforms for better working conditions as well as help return refugees to their countries
negative effects
Now as far as the negative effects, the League of Nation's lack of having a strong backbone and enforcement is it's ultimate downfall in preventing wars and resolving disputes. The League's failure in the Manchurian Crisis as well as the Abyssinia Crisis against strong forces of aggressors causes it to lose reputations and trust from other nations. In doing so, the League's failure became a stepping stone for Hitler because it prove to him that he could get away with aggressive actions and thus supported Hitler rise to power. Evidently, this will lead to WWII. "As a result of the League's clumsy handling of Italy's invasion of Ethiopia (angering Italy for the League's condemnation, but doing little to actually stop Italy) Italy and Germany signed a treaty of friendship on October 25, 1936. Mussolini commented that the two countries had created a Rome-Berlin axis around which all European nations would eventually revolve. Thus the term 'Axis Powers' came into usage." ( Source: Kingsacademy.org)
Hitler's Speech:
"History proves: He who has not the strength - him the 'right in itself' profits not a whit. A world court without a world police would be a joke. And from what nations of the present League of Nations would then this force be recruited? Perhaps from the ranks of the old German Army? THE WHOLE WORLD OF NATURE IS A MIGHTY STRUGGLE BETWEEN STRENGTH AND WEAKNESS - AN ETERNAL VICTORY OF THE STRONG OVER THE WEAK. There would be nothing but decay in the whole of Nature if this were not so. States which should offend against the elementary law would fall into decay. You need not seek for long to find an example of such mortal decay: you can see it in the Reich of today...."
- Adolf Hitler
In his speech in Munich on April 13, 1923, Hitler criticizes the League because although it's known to be the world police, it has no strength whatsoever, clearly proven through the events of Manchuria and Abyssinia Crisis. He insist that there is no order or regulation, it's basically the principle of survival of the fittest. Only the strongest countries can thrive.