Most my source material here is from the Literary Digest's First Volume of the war's history.

I would argue that a war between the major European powers was inevitable, but also that WWI as it happened was not. There were several crises in Morocco and (I think) Sudan that could have started the war on their own with slightly different alliance structures. The debate over French and British colonies in East Africa came pretty close to this, and would have had the French and British fighting each other.

Moving on to the Balkans, I believe the general consensus is that Germany escalated the crises after Ferdinand's assassination, thus causing the alliances to fight each other. That doesn't mean that the Balkans or assassination were unimportant, just that Germany saw an opportunity to defeat other European powers and elevate their position in the world. According the the Digest's account, many Germans felt that their country's potential was being held back by Britain and France. They were industrial and military leaders, but came too late to the imperialism game to really get any worthwhile colonies. Remembering Bismark's realpolitik ideology, the Germans felt that the best colonies should go to the strongest country and were willing to fight for that title.

The assassination also played into Austrian desire to control the Balkans. After the fact, the Austrians went back and forth on whether or not to declare war on Serbia. They offered an ultimatum with multiple points, and Serbia accepted all but one (something about Austria leading the investigation of the Archduke's assassination). This was the direct reason for the war starting. Would it have happened without German support of Austria? It's hard to say, but given Austria's relative weakness at this point, it's tough to imagine them risking a war with half of Europe.

The Balkans may also have been important because of its position on the Mediterranean. Control over these countries, combined with the alliance with Italy and the later alliance with the Ottomans, would have given Germany access to a massive part of the Med. Since it's always been sort of the backbone of the Royal Navy's strength in Europe, this presented a problem for the Brits. I've never seen this part of the pre war phase explored by historians, but it's interesting nonetheless.

So to round it off: the war was probably unavoidable, but could have happened in different ways. Germany escalated the problem in the Balkans and the alliance system in Europe meant that the (almost) the whole continent got involved.