In what ways did World War I represent a frightening new kind of warfare?

WW1 was the first war to abandon the line styles of "Noble Fighting" found in contemporary Europe. In the old style soldiers walked in line and proceed to shoot each other till the other fled or simply all died. In the horrific new war trenches were dug and six machine gunners could hold off and assault of thousands of contemporary linemen. Doing so led to the horror of trench war which was bloody much more violent and dark.

Principally in the use of gas.

Aerial bombing was still in its infancy, though the newly-formed RAF Bomber Command was planning its first big raid on Berlin for November 11th 1918. It had to be aborted when the whistle went.