2-D VS. 3-D
One of the biggest differences between hand-drawn animation and computer animation is the fact that computer animation is truly three dimensional. The first run cycle I ever animated on the computer looked great from the side view, but when I looked at it from the front, the arms were going through the body and the knees were bending the wrong way. From then on I always animated with two views of my character always showing, so that I could always tell if the animation was working from all sides.
Since computer animation is truly 3-D, you can re-use the animation of a scene or parts of the animation and not tell that it is the exact same motion. Many times, if you simply look at a scene of animation from a different camera angle, it will look completely different. Sometimes, it may seem too similar, so just vary the timing of the motion or change the motion of an arm or head and it will cease to resemble the original. This is great for crowd shots, where the re-use of animation is an easy way to keep the crowd alive.