Wars are incredibly devastating and brutal things. This is a simple truth and I find it somewhat odd that people think the Great War was somehow an anomaly in this regard.

To quote John Terraine in The Smoke and The Fire

No one questions the Duke of Marlborough's claim to be considered a 'great captain' - despite the fearful losses of Malplaquet. The Duke of Wellington, in Spain and at Waterloo, presided over scenes of awful carnage -bad enough at Waterloo to bring tears to his eyes. Many of Napoleon's battles were sheer blood-baths. General Robert E. Lee, often considered an outstanding military genius, never hesitated to accept heavy casualties in the pursuit of victory. The Soviet marshals of 1939-45 were undismayed by the veritable massacre of their men in the same quest - but nobody hounds their memories as the generals of the Great War have been hounded.

The actual casualty rates of WWI were unremarkable. They were equalled and exceeded before and have been equalled and exceeded regularly since. Even the casualty numbers, in absolute terms, were easily exceeded by the second war.

Its simply a matter of perception, particularly in the English speaking world, that the Great War was somehow unique in its horror and carnage, and that's simply because it was the war in which the English speaking people played the largest role - that of engaging and defeating the main body of the main enemy on the main front of a general war, and therefore paid a price commensurate with that role.

The point /u/jonewer makes below (a point made by John Terraine), is essentially about casualty rates. Casualty rates in the Second World War, especially on the Eastern Front, and on the Western Front from 1943, were pretty high. The only difference was, the Armies of the Second World War possessed the mobility and ease of communications that the Armies of the First World War did not, to restore decisive action and high tempo to operations.

Battles before WWI were quite bloody as well. Borodino, Waterloo, Leipzig, Antietam, Shiloh, Manassas, Gettysburg, Sedan, Gravelotte & St. Privat all saw bloody fighting, and heavy casualties. The only difference was that the size of the armies involved and the duration of these battles was nowhere near that of WWI; as Ludendorff predicted before WWI, the next war would be "a long-drawn out campaign with numerous difficult, long-lasting battles, before we can defeat [even] one of our adversaries." With more advanced technology and bigger, better armies, multiple Borodinos, multiple Waterloos, and multiple Gettysburgs would need to be fought before either side came close to reaching a decision.

By 1916-17, Germany, France and Britain had more or less developed the industrial and military means to prosecute lengthy Materialschlacht ('Battle(s) of Materiel') like the Somme and Verdun, but to reach that level of preparedness would have required state-intervention, military and industrial expansion, and immense taxation, conscription and budgetary expansion that would have been unthinkable to liberal and conservative minds alike prior to WWI. [Nicholas Murray raises an excellent point in this lecture] (https://www.youtube.com/watch?v=2cbq7iu8FrI) about what a Commander in Chief may have known he needed, and what the 'Chancellor of the Exchequer' would have believed feasible in peacetime.