To a young person of the current generation (especially in the west), the most eye-catching thing might be the clothing. The Ushanka-like caps, the medals on men and women, the military uniforms. Next, you notice the stern-looking man in the center.
Theoretically, you might expect this man, being the main subject, to be in the middle, but he is not. He is slightly to the right. This creates a sort of "tension". Because our brain almost expects the man to be in the middle, the fact that he is not makes not be able to look away.
But there is so much more. Notice how the people are all (or mostlly) looking in the same direction. What are they looking at? These few details are already telling the viewer a story.
The way the people are placed in the crowd, and the position of the photographer that took the picture creates the illusion of a triangle, again drawing your eye to the old man.
At the same time, one other detail that stands out is the person on the bottom right corner. The one person that is NOT looking to the front, who is also dressed in white, contrary to the rest of the crowd.
One last thing. This happened to me and might have happened to you too. The horizon is not level. You can see the background clearly askew. But you don't even notice it at first, beacase your eyes are drawn to the details I mentioned. And that's when you realize what a great photo this is.
This photo was taken by Henri Cartier-Bresson in Moscow, Soviet Union, in 1954.