The weakening influence of the Concert of Vienna in 1815 and the growing European Nationalism and Imperialism were the two causes of World War I. The peace established by Europeans after the Napoleonic Wars lasted for almost exactly a century, but during that time, the various nations of Europe were all in a race to establish national sovereignty, which unfortunately included the establishment of colonies around the world. The constant clash between colonies led to the inevitable clash between the European mother countries. Assassinating the Archduke was Serbia's attempt to break away from Austria, which was the final event that brought the major powers to a standoff, the so-called "spark in the powder keg."
Behind even what Post 2 is talking about, we need to look at the fact that countries seem to desire power and to fear what others might do to them. They are trying to make themselves stronger (as with Bismarck) and these actions make other countries afraid. This leads to a spiral effect where Germany, for example, tries to get stronger which frightens Britain and makes them get stronger, which in turn frightens Germany.... This sort of a spiral helped cause the war.
I agree, many of the long term causes had been almost 50 years in the making. Even the famed alliance systems, which ensured that the conflict in the Balkans would become a wider European war, had their roots in diplomatic maneuvers by Otto von Bismarck that were aimed at avoiding a Franco-Russian alliance. Obviously the issue of nationalism within the Austro-Hungarian Empire was not a new one, and the roots of Anglo-German competition, especially naval competition, went well back into the nineteenth century. Events as cataclysmic as WWI seldom have simple causes.