I would definitely say that Nationalism was the greatest cause. Beyond all the great reasons listed above, the Slavic peoples desire to be a part of Serbia rather than the Austro-Hungarian empire led to the social unrest in that region that eventually led to the assassination of Archduke Franz Ferdinand, which put into motion the path of war commonly accepted today.
In the absence of Slavic nationalism it is possible that there would not have been an assassination and perhaps not a war.
Nationalism motivated the populations towards war, but mass naivete in each country (except perhaps Russia) about any sort of quick or bloodless victory was the more important factor in my opinion. You can see this in the excitement with which the populations embraced the onset of war, and the number of citizens in each nation that quickly volunteered for war. It should be said, though, that this is not unique to World War I or any particular country. The US had strong popular support for the Vietnam War for the first four years in a naive belief that historical factors and a resilient insurgency would be so simple to beat.
An important factor in the uprising of WWI was the failed role of diplomacy. By definition, diplomacy is the managing of international political relations. Prior to WWI, European diplomacy had descended to manipulating international relations instead, and this brand of diplomacy failed egregiously.
I like the idea presented in #2. There definitely was a feeling that the war would be over by Christmas and would be just a very short-term thing. If the various leaders of the countries involved knew just how painful and how tragic and drawn out the war was going to be, I think they would have had very different ideas, responses and attitudes towards it.
Another factor was the rise and relative success of the United States; nobody in Europe thought the experiment would last, since it focused on individual freedom and personal responsibility instead of God-approved monarchy or dictatorship; as news of U.S. success spread, people in the Royalist, Fascist, or Socialist European societies became more and more dissatisfied with their lot.
The clash of competing empires around the world eventually came home to roost in Europe. All the aforementioned are factors for describing the onset of the war, but the competition between various colonies abroad eventually brought about domestic conflict. World War I could also have been seen as the final breakdown of the Concert of Vienna which had held Europe in peace for a century after the Napoleonic Wars. Clearly mass industrialization and the social dislocation it caused were major factors in altering the established order in the 1800's, the European Imperialism that resulted caused not only a conflict of markets, but a conflict of ethnic groups, all of which demanded self-determination. It appears there were forces within and without Europe challenging the established order, both domestically and internationally, which simply could not be satisfactorily addressed without warfare.
I don't see where isolationism played much of a role, exactly the opposite really- the nations of Europe were bound together by military alliances. I would suggest that it was the incapacity of military and political leaders to avoid war that brought it about. The officer corps in Germany, Austria-Hungary, and even Russia each encouraged aggressive stances in the wake of the crisis in the Balkans, and their belligerence, unchecked by monarchs, intensified the crisis.
Nationalism definitely played a part. There was a great deal of nationalistic response to trouble in the imperial governments, the economy and the relationships between countries. Isolationism and nationalism are a troublesome combination. This led to the conflict between countries in World War I and contributed to World War II also.
Of these choices, I'll go with mass naivete. The reason I say this is because people in every country seemed to think that the war could be quick and relatively painless. They cheered the idea of war, thinking it would be glorious. If they had realized what was coming, the war might not have happened.