One key element is the transformation. Something or someone changes in an unexpected and often frightening way. Another is the sense of the ordinary turned extraordinary. The lawnmower itself is probably an important element as it's in the title and likely plays a central role in the horror that unfolds.
The key elements in Stephen King's 'Lawnmower Man' are multi - faceted. The concept of the familiar becoming alien is a major one. We have the lawnmower man, who represents a common figure, perhaps someone we might see in our neighborhoods doing his job. But then this normalcy is shattered. There is also the element of the unknown. King doesn't reveal everything at once. He keeps the reader guessing about what is really going on. The horror in the story might be related to some sort of dark force or transformation that defies our understanding. And the way King describes the environment, the sounds, and the actions all contribute to creating a sense of impending doom that is central to the story.