i dont think it was the alliance system exactly, but the way all the countries handled their allainces. Before 1914 the countries used their allainces to pressure there allies into avoiding war, while in 1914 countries like Austria and Russia used thier allainces to pressure Germany and France INTO a war. Also, before around 1905 (Russo-Japanses war), the way war was conducted prevented such a long conflict from occuring. There were wars (Franco-Prussian, Six weeks war, Crimean war, etc.) but they were wars of movement and armies, not total wars of trenches and entire nations.
Im officialy a comrade now, pretty sweet. Fear my bolshevik hordes! 8)