Other conflicts to learn from are probably the many smaller colonial wars (about which I don't know enough to really comment)
Well, the British army of 1914 had a strong emphasis on teaching its troops fieldcraft and marksmanship skills, a direct result of their experience against the Boers. This emphasis stood them in good stead in the early months of WW1.
I guess Europeans thought the American Civil War lasting 4 years must have been an American phenomena then? Because WW1 was also expected to initially be a short war.
I don't know what, if anything, Europeans of the 1910s thought about the ACW, but the reason they expected WW1 to be short was because they were basically expecting it to follow the pattern of previous European wars, namely, "Both sides gather as many troops as possible and march against each other, you fight a big battle, and then either the loser sues for peace or the winner pushes onwards, keeping the loser off-balance and driving them back until they sue for peace." That's probably how the ACW would have gone if both sides had started off with big European-style armies, too: if the Confederates had been well-trained enough to keep their cohesion after Bull Run, they could have pushed onwards immediately, scattered the retreating Union Army, and attacked Washington before the Feds could get their defences in order.