In what ways was the League of Nations unjust?

The greatest success of the League of Nations is that it served as a precursor to the United Nations. Its greatest failure, however, is that it failed to keep the peace among nations, as evidenced by the rise of the Third Reich, resulting in World War II.
Primarily, historians consider the League unjust due to its punitive approach toward Germany. The Treaty of Versailles, where the League of Nations was formed, disarmed Germany, levied large debts, and redistributed territory as reparation to other nations for World War I. While reparations could be considered appropriate, these sanctions hobbled Germany's ability to recover from the war in the homeland.
In addition, the League singled out Germany as ineligible for membership even though other powers who fought alongside them (Bulgaria and Austria-Hungary, for example) were permitted to join. If the League's stated goal was to maintain peace among nations, giving Germany a voice would seem essential. Germany was later allowed to join the League in 1924, but Germany withdrew after Hitler's appointment as chancellor.