The story achieves its impact by being understated and matter-of-fact. There is a ritualistic murder that takes place every year, but how the people go about it is what is most disturbing. Let me give you a few examples.
First, the children are involved. They just finished school and are about to enjoy the summer. When the day of the lottery comes, it is as if it is an ordinary day. They will partake in stoning someone and not bat an eye. Then they will go on playing, even though one member of their small community will be killed by stoning.
Second, as the fathers come to the lottery, they talk about work. There is small talk - no feelings of horror or dread. This is also true of the mothers, who are catching up on the latest gossip.
In light of this, the very nonchalance of the village adds the biggest impact.
Here is a quote. In my opinion, it is one of the most disturbing:
The children assembled first, of course. School was recently over for the summer, and the feeling of liberty sat uneasily on most of them; they tended to gather together quietly for a while before they broke into boisterous play. and their talk was still of the classroom and the teacher, of books and reprimands. Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones...