So far I have learned that, perhaps, the most important principle is "Anticipation." I had originally had the anticipation cell wrong. Of course, I was able to use the "cell," but what we discovered was that it was not the right cell to really deliver the proper anticipation, the pose was not extreme enough to be the critical anticipation cell. The solution was to create a new anticipation cell that was far more extreme than what I had initially imagined. The other key element was not only the pose itself, but timing.
The relation between the anticipation cell and the action itself is also important. Big actions require big anticipations, at least, to my knowledge so far.
Timing has also been of critical importance. Timing alone can also make or break the believability of an action... even if the drawings are all spot-on.
Research as also been terribly important. Making shit up just doesn't work. It never has and never will, and when trying to create believable motion, it is of major importance. Research should be the 13th principle.
The research I have used so far has been:
1) Viewing all the falling, jumping, and landing sequences I could in normal and slow motion speed, in live-action films as well as animated movies of various qualities.
2) Practicing the action itself in the physical world.
3) Looking for still images of the motion I am trying to recreate.
4) Creating test-loops to study my animation.
5) Getting a second opinion.