On the Western front it is undoubtedly the extensive trench network. The armies had fought each other to a standstill by the end of 1914, the use of modern weapons and artillery necessitated "digging in" and taking up defensive positions. A trench line was created all the way from the Channel Coast to the Swiss border.

As commander of the British Expeditionary Force, Sir John French wrote towards the end of 1914: “modern weapons and conditions have completely revolutionised war. It is completely different from anything which… I have known. A battle is a siege on one side and a fortress defence on the other, but on a gigantic scale.”

Of course trenches were not new, they had been used to an extent during the Boer War, and to a far greater extent in the Russo-Japanese War of 1905, but these were as in the Boer War niche examples, or in Russo-Japanese War a product of the overall objectives (siege of Port Arthur). With the Great War the whole line had become static, and could seemingly only be shifted by greater quantities of heavy artillery. Much of pre-war thinking was focused on mobile battlefields with decisive actions, such as in the Franco-Prussian War of the 1870's.

The idea of a long drawn out struggle seemed contrary to the destructive power of the weapons employed. The tactics of the combatant armies developed in the inter-war period had not come up with an adequate solution to cross the fire swept zone, particularly where static lines and heavily entrenched positions were concerned. The prevailing belief was that men must move as quickly as possible since the fire swept zone was imagined as a sort of death zone which had to be escaped. Combat doctrines entering the Great War were heavily weighted towards offensive actions, and did not really consider attritional warfare focusing instead on taking ground which proved very costly due to the parity of the combatants.

One of the developments during the 19th c. was industrial manufacturing, where new energy sources and technological developments made it possible to create very large factories. Combined with improved financial systems and advanced transportation, this lowered the labor needed and greatly lowered the cost. The result was a huge increase in production of goods, exemplified by Henry Ford's giant Detroit plant turning out Model T cars at $500 each.

While there were some industrial aspects to the US Civil War, WWI is really the first industrial war. Millions of artillery shells were made, shipped, delivered and fired. Millions of rounds of rifle ammunition were also made, and distributed over battlefields by rapid-fire rifles and machine guns. Armored vehicles were developed, and combat airplanes. And in the face of such enormous forces, the long-standard use of human infantry to take- and hold- territory became much more difficult. In the pre-industrial war, a company of valiant soldiers, filled with élan, might storm forward with a shout and overwhelm the defending line. In the new industrial war, that charge could be stopped by a machine gun and a few defenders. Like Henry Ford could turn out a million cars, the new industrial war, employing the same huge improvements in technology and manufacturing, could mass produce death and destruction.

This was in contrast to the war experience of the combatants. England, France, Germany and the US in the previous decades had been able to successfully employ their armies mostly in colonial wars, against far less powerful opponents. This success gave them a false notion of how successful, and how limited, a war would be, gave them the idea that it would be glorious, useful. They were therefore more willing to start it.

WWI has been a very big topic for historians. It's awfully important, you could say it was the beginning of the modern world. You'll find some good books on it over on the Bookshelf.