I think Sir Michael Howard put it best:

"Wars would be BIG, BLOODY, and DECISIVE"

The Franco-Prussian War, The Russo-Japanese War, and the Balkan Wars all seemed to suggest this. Casualties would be high, but then they'd always been fairly high in the Napoleonic Wars, and the Armies were larger now. Wars would be big, but they would utilize railways and aircraft to move armies swiftly and obtain Intel on advancing armies.

Whoever mobilized first and the fastest, and brought the largest, most well-armed, well-trained and most of all well-motivated (spirit of the offensive) Army to the field, would triumph. That's pretty much how the Prussians had won the Austro-Prussian and Franco-Prussian Wars, and how the Japanese had won the Russo-Japanese War. Wars, then, would be DECISIVE.

In the end, WWI was two out of the three: it was BIG, it was BLOODY, but it would be 4 years before it was DECISIVE.

Geoffrey Wawro, "Mad Catastrophe"; Michael Howard, "A Short History of WWI"; Prit Buttar, "Collision of Empires"; Hew Strachan, "The first World War, Volume One: To Arms!"; Peter Hart, "The Great War: A Combat History of the First World War"; Max Hastings, "Mad Catastrophe"

Did they think the war would be short? In many respects yes and they had good reason to believe so...sort of. It depends on which front we are speaking about and from whose perspective. To sweep over the 'general':

• The Austro-Hungarians felt the war would be rapid w.r.t. Serbia. They greatly overestimated their ability and assumed Serbia would be, in essence, a walkover. Serbia on the other hand wanted just what they got; to attrit the Austro-Hungarians, reduce the fighting spirit, hope that through long enough suffering the internal national divides would cause a breakdown. More or less happened exactly like that too until Bulgaria entered the war.

• The Russians and French thought the war against Germany would be rapid. Plan XVIII, accepted by the French Generals Staff in 1913, was the culmination of almost two decades of military planning. The plan was to play to both Russia and France's strengths and alleviate their weaknesses. With a roughly equal standing army (but vastly outnumbered reserve armies) the French were to strike hard and fast across the Franco-German border while parity still existed. Simultaneously the Russians agreed that despite the fact their mobilization would take a significant period of time to be able to contribute ~700,000 men in an incursion into East Prussia on the start of the war. The plan was that the Germans would be overwhelmed from all angles as you might imagine. In the event of the wars continuation they had the British on their front to take over and the Russians in the East to out attrit the Germans.

• The Germans thought the war would be over quickly in France and their plan called for their surrender in under 40 days. However they believed the war in the East would be more or less how it was; a great struggle that would last many years. They recognized the Russian superiority when fully mobilized and wanted to slay the French beast while she was isolated and then the combined German, Austro-Hungarian, and Ottoman forces would 'encircle' Russia and squeeze them dry.

• The British very quickly recognized the reality of a long term war. While they were always about continental intervention it would only be until 1916, at the Somme, that they would in good faith fully get involved in the land war in Europe. Their plan was one of support and long term breakdown than anything else. Strike at the Ottoman fringes in Egypt/Palestine/Mesopotamia, blockade the Germans, etc. They tried to break the stalemate through Gallipoli but it was also a long term plan; the hope being that seizing Constantinople or just causing enough damage would let the Russians thrive and cause a cascading effect. Turks fall, Bulgaria joins the Allies, Austro-Hungarians are attacked on all fronts and Russians are empowered, A-H falls and Germany is isolated and beaten. So the Brits really always had long term plans in mind.