When you design the most important part of your design is the Silhouette.
The silhouette should be:
1. Simple Iconic and memorable: Be appealing.
2. Communicate: Define the emotional tone of your character (even if the interior details and color are blacked out).
To complex and forgettable.
Just right ;)
(I believe Bill Perkins is responsible for this famous sheet. )
2. Communicate clearly:
There three basic emotional tones any piece of illustration or design will emote. Feelings of:
Stability, Rest, or Movement.
Circles tend to emit a feeling of REST.