WWI was the first war many modern states had fought against eachother in decades. The British had engaged in some guerilla fighting with the Boers at the turn of the century, but for a 'proper war' you really need to go back to 1871 for France and Germany, 1865 for the US and 1856 for the UK.

In 1914 the British army didn't issue helmets as standard, and the French still wore blue uniforms; German troops actually went into battle with little spikes on the tops of their helmets.

WWI standardised helmets, camouflaged uniform and regular issue of gas masks. It taught officers the importance of toning down their rank insignia, with many wearing other ranks' jackets (and the British in particular, removing the highly conspicuous cuff insignia that made them sitting ducks for snipers).

I suppose the sheer numbers of troops deployed, together with what was a completely new experience, worlds away from the imperial wars many officers and senior NCOs had cut their teeth on, pushed the necessity for a uniform that was cheap and effective. Each country did it slightly differently, but ultimately ended up largely in a similar position.

There is an argument that 'modern' warfare started at different times in different countries, and that many armies actually forgot lessons learnt. The US (and CS) armies in 1864/65 arguably used more advanced tactics - loose formations, heavy skirmisher screens, officers blending in with their troops etc... - than the doughboys who, in 1917, marched into battle as if they were on the parade ground.