the principles of animation:

1. Squash and stretch

2. Anticipation

3. Staging

4. Straight Ahead Action and Pose to Pose

5. Follow Through and Overlapping Action

6. Slow In and Slow Out

7. Arcs

8. Secondary Action

9. Timing

10. Exaggeration

11. Solid Drawing

12. Appeal

explained:
-a ball will squash as it hits the floor and stretch slightly as it comes back up from the ground.

-before you jump you duck and tuck your arms in.

-staging
the angle you see somthing makes a huge difference to the emotion of that character or that scene.

-straight ahead vs pose to pose
animation is animating frame by frame or in cg terms setting keys as you go rather than setting poses at each stage of the scene
pose to pose is working litterally from key pose to key pose rather than running straight through your scene. Here often flat tangents are used in the graph editor.

-slow in and slow out
a character will not move evenly, if somone is kicking somthing their foot will slow in and out of the kick.

-arcs
nothing except machines move in straight lines, add arcs to the motion of every object in your scene, be it a hand or a ball.

-secondary action
when someone is talking their lips dont just move they may signal somthing else with their eyes or hands, this is essential for belivable character acting.

-timing
duh, the time that somthing happens plays a huge influence on the drame of the scene, if a character picks somthing up too quickly it can look like a grab rather than simply picking somthing up.

Exhaggeration
-real life movements are far too subtle and detailed to capture realistically, especially when talking about facial motions or subtle jestures, to make a point more obvious you exhadurate motions.

-solid drawing doesnt really apply, but if somthing is too weak in line quality or has a messy mesh etc the aimation is not clear, as mckinley said rig quality and mesh detail is the equivilant to this.

-appeal -a good pose is allways readable as a sillhouette you an see, hello alpha channel