Why was WWI considered a new kind of war?

Already in 1914, at the beginning of war, battle was much more violent than it had ever been before. And then military and civilian suffering gradually intensified the violence over the duration of the conflict. This progressive intensification lent its own dynamic to the conflict; in the very first days and weeks of the war the practices of war took a brutal turn, not only on the battlefields but also for prisoners and civilians. Even for the ordinary soldier, the enormous explosion of violence that occurred in the summer of 1914 immediately and scathingly refuted all the predictions that had been made in the years prior.

The death toll of the Great War is well known: around 9 - 10 million, nearly all soldiers

So basically they had a new kind of technology to make the war more deadlier