FTS
We could get into considerable debate on this one, in which firearms buffs (such as myself) would be in their element!
Between the American Civil War and the outbreak of WWI, weapons technology took a quantum leap. During the Civil War, the standard weapon on both sides was still the muzzle-loading percussion rifle (although the Confederacy entered the conflict armed with the even older flintlock muskets). Machine guns were not used in the Civil War. Gatling developed his hand-cranked gun during that war, but it never saw service until afterwards. Even then, the world's first truly automatic machine gun was still 20 years away (Hiram Maxim's invention). The problem with the Gatling, Gardner, Nordenfelt, Mitrailluese and other hand cranked weapons was that in the heat of battle, the operator often panicked and started cranking the weapon too quickly. This resulted in its jamming. Another issue was the propellant used in the cartridges - still the charcoal / sulphur / saltpetre blend of gunpowder. That added to the jamming issues, and obscured the battle field in a great fog of gunsmoke. This same problem delayed the world's armies from adopting magazine fed rifles until the 1890s, when smokeless powders were developed.
Artillery, also, took quantum leaps during the latter part of the 19th Century, thanks to the arms race initiated by the German manufacturer, Krupp.
Likewise for naval development. Naval warfare was revolutionised in 1906 by the launching of HMS Dreadnought, which rendered all other battle ships obsolete. That set going a massive arms race - principally between Germany and Britain - which was in full swing by 1914.
Add to the list aircraft (invented 1903), poison gas, submarines, torpedoes and mines (both of which changed the balance of naval warfare), depth charges, sonar, sound-ranging equipment, tanks, armoured cars etc, and you come up with a lot of things never used previously in an all out conflict between major powers. It is true that a number of these weapons (but by no means all) had been experimented with previously in small scale Colonial wars. However, in those days before Satellite TV and the like, it would have been a case of "out of site, out of mind". World War One brought home the horrors of a modern, industrialised war to the heartland of Europe.