Why is it deadly to hit concrete when falling from a high place?

This one goes back to a central concept in physics called the impulse-momentum theorem. The impulse momentum-theorem relates a change in velocity with the force that causes this change in velocity.

Warning: What follows is a little graphic.

Say suicidal Johnny leaps off a tall building. On his way down, Johnny speeds up and gains a lot of momentum. When he hits the ground below, he stops suddenly. His momentum goes from a large amount to zero. This happens because the ground exerts a large impulse on Johnny. That impulse is equal to the force the ground exerts on Johnny times the amount of time it takes for the ground to stop Johnny. The latter is very short, as unlike a mattress, concrete is hard. Johnny stops over the course of a few milliseconds. The force from the ground has to be very large in order to compensate for the tiny time interval and still provide the impulse needed to stop Johnny's fall. The force is so large that it causes all kind of damage to suicidal Johnny's body.

Related post: How do ducklings get away with falling from great heights without getting hurt?

Why do golf balls have dimples?

Golf balls have dimples so that they are less sensitive to air resistance and go further when hit. You might think air would flow more easily around a smooth ball and so such a ball would cut through the air more easily. But that is not the case.

A smooth ball carries around itself a pocket of still air. Around that, there is turbulent air. A small bug on the surface of a smooth ball is actually protected by this thin layer of thin air and will not feel a wind as the ball flies. This layer of thin air increases the effective size of the ball and a larger ball has a harder time cutting through air. It will not be able to fly as fast and won't go as far.

A dimpled ball creates air turbulences around itself as it cuts through the air and so it doesn't have this handicap of looking like a bigger ball. It cuts through air more easily and goes further.

Reference:
The Physics of Baseball: Third Edition, Revised, Updated and Expanded, 2002, Robert Adair