If the bars are too low or have too much sweep, this will force the riders elbows inward to compensate for the awkward angle of his wrists. It also forces the rider to sit rather than stand.
Notice how the rider is in a more neutral position on the bike, with elbows up and his arm and wrist are in line which offers more strength to absorb impacts when landing jumps or hitting obstacles.
This set-up is forcing the riders upper body and elbows to sit up high. This makes it hard to get your body into the bike when cornering and will tire the rider quickly.
Again, this image displays the bars too high (when standing).
If the bars are over-rotated in the clamps (like in this image), you can be putting too much body weight over the front end of the bike and effect the handling and stability of the bike.
In this image the bars are too low.
With the rider standing, you'll notice that the riders wrists are bent at an uncomfortable angle. This is putting unnecessary strain on the wrist joint.