1.) Timing
Timing for each frame in a animation is crucial to create a sense of movement. This all depends on what type of speed you would like your scene to be. If you needed a super slow-motion matrix scene, you would want to create several drawings to create multiple frames. If you would want to create a fast paced action scene, you wouldn't create as many drawings. Try to compare the speed of movement in reality to match in a animation. Most animations are done in twos (each drawing is duplicated to create 2 frames). Most animations are also done in 12 fps (12 frames per second), but there are many other artists who create their frames in ones (individual drawings for each frame), and creates animations in 24-60 fps!
2.) Solid Drawing
A solid drawing can be created by giving your animation life. Solid drawing wouldn't be a 2-D drawing. Solid drawings are 3-D or 4-D drawings created from the space they are placed in. 3-D meaning having a 3 dimensional illusion creating depth into a animation. 4-D means crating a animation that works with the whole environment in the animation.
3.) Appeal
In order for an animation to look interesting, the theme, characters, objects, etc, must look "appealing". A good way to make your animation to look appealing is to creating a interesting story with great animations to match. Uniqueness can also bring a viewer interested into watching your animation.
4.) Exaggeration
Exaggeration focuses on the expressionism of animations. Exaggeration distorts them expressions of Instead of showing the mood of someone with a happy expression, it wouldn't just be a smile on their face. The environment around them may show bright colours to show happiness as well. Exaggeration are also very "overly exaggerated". Here is a video of Exaggeration:
