Weaponry and defence engineering but perhaps above all the mobilisation - and vulnerability to effective food-supply blockade - of the "home front" which engaged whole populations that might otherwise have largely been left relatively undisturbed.

This was perceived as the first "total war", explicitly encompassing not just those on the front line but the whole of society, at least among its most active participants. To that extent it prefigured the exposure of civilian home populations to military action.

The conversion of whole national economies to support of armed endeavour was quite unprecedented, even if Britain and Germany had previously entered into competition for naval power with significant domestic political consequences in the first case.

A "people's war" it may not have been, the common people emerging no better off at its end. But it reached far beyond the traditional clash of armour, leaving Europe poorly placed to recover its past re-eminence.

To that extent it was modern in denying real winners.

In brief it was due to three main factors - industrialization, mass mobilization, and yes, the introduction of considerably more 'modern' weapons.

Industrialization There's no use in raising a large army if you can't feed and transport them. This was one large constraint in the Napoleonic Wars, and a sharp contrast to the U.S. Civil War, where large0scale use of railways for supply allowed army sizes to soar up to the point of the Union having half a million men under arms.

By 1914, Western Europe was crisscrossed with an very dense rail network that was to prove crucial for both sides, with the German war plan in particular moving more than half a million men to the Belgian border by rail within weeks of announcing a full mobilization. Supply by rail allowed for similarly profligate use of shells, with the British preliminary bombardment at the Somme going on continuously for days, and for machine-guns to spew forth death with little thought for supply. Lastly, mechanization and modernization of agriculture allowed many countries to generate a surplus of food. This, together with the global trade network engendered by the Industrial Revolution allowed the Allied armies to be supplied with American food, Indian rubber and troops, Middle Eastern oil, and other such produce form around the world. Industrialization also allowed the Germans to produce similar 'ersatz' substitutes for lost colonial produce, such as nitrates form Nauru.

Mass Mobilization This generally refers to the gigantic size of national armies, culminating in the first use of the 'army-group' level of organization. Additionally a far larger amount of men were mobilized in many countries, with Germany calling up a total of 13.25 million men (Figure from ref.1) over the war. Similarly one sees the sheer size of armies allowing for a truly gigantic reorganization of the homefront with rationing, women working en masse, scavenging of raw materials (Many roofs lost copper for drive bands, for instance), and a gigantic retooling for war production, especially shells and ammunition.

Weapons From the Civil War with the use of the rifled musket, and the mitrailleuse, chassepot, and steel breechloading cannon of 1871, the First World War saw the first use of gas (Ypres), the first use of tanks (Cambrai, 1915), and the deployment of machine guns at a far larger level than prewar. A prewar British infantry battalion had two MGs, and four by 1915. By 1918 this had greatly increased. Lastly, the rifles themselves were far deadlier, with a longer range and high accuracy in the right hands - one such example is the famous marksmanship of the old British Army, trained to fire (twelve? I'm not sure on this detail) aimed rounds a minute.

Sources- 1)Paul Kennedy The Rise And Fall of the Great Powers

2)John Keegan The First World War

If you define "modern war" as nuclear weapons, then there have been no modern wars except the last few weeks of World War II. If you define "modern war" as international organizations participating, then World War II was not a "modern war" either. So I am not sure either of these are great criteria.

How you define" modern" will determine what answer you get to your question. I think "nuclear weapons, religious/political movements, international organizations" is a pretty arbitrary set of definitions of "modern." The other answers here give indications of other definitions of "modern," which may be equally arbitrary. The thing to keep in mind is that there is no fixed definition of "modern" and necessarily it will look different as time goes by. You can construct a definition of "modernity" that includes World War I, you can construct one that excludes it, you can construct one that sees World War I as a catalyst, etc. There is no "natural" definition of "modernity" — it is an analyst's category. So if you want to adopt one definition over another, you have to argue for it, and what it gets you to use your definition versus another one.

I prefer something like the "catalyst" definition: World War I is the beginning of many trends, but it does not itself exemplify all of them. It was a war dominated by the forces of industrialization of military production and the specter of new technologies; it was a war that set off the rise of political groups with radical ideologies on both the left and right extremes of the political spectrum; it was a war that both splintered and re-made nationalisms on a massive scale; it was a war that made generals realize there was much more than tactics at stake for winning large-scale victories; it was a war in which many nations felt, not incorrectly, that their existences hinged on victory and defeat. These sorts of trends would be extended and amplified into the Second World War and beyond into the Cold War, and arguably today. This particular definition emphasizes that in many ways World War I is not so different from wars that came before (which would make sense given that there are preconditions for each of these aspects), but that it does seem to mark a demarcation between the more Napoleonic-style wars of the previous century and the highly-mechanized and quite global conflicts to come in the 20th century.