The Most Important Aspect Of Animation Is...

Motion!

Okay, that's a cop out. Saying the most important part of animation is motion is like saying that the most important part of a TV is the screen. Animation IS motion, so motion is the most important part for a good reason - once motion ceases, it is no longer animation. Motion could also be thought of as the activation of time in a visual medium.

But motion comes in 4 flavors.

1. Actor/Object - Balls bouncing, Mickey running, Gumby doing a big stretch, things that activate time in the 3rd person (effecting the characters) visual sense.

2. Camera - Sweeps and pans, things that activate time in a first person (affecting the viewer) visual sense.

3. Sound - BANG!, "Hello, I'm Artemis!", or even CRASH!, things that activate time in a third person aural sense.

4. Incidental Music - Probably the William Tell Overture, things that activate time in a first person aural sense.

I would argue that, unless at least two of these types of motion is happening, you do not have an interesting animation, while unless at least one is happening, you cease to have animation at all, and revert to still visual art, as in photography or painting.

So, being that motion is the essence of animation, and the only thing that, when removed, also removes the notion of animation, it's pretty hard to argue against it's importance.