Much like the OP, I am ending a long period of lurkdom with this here post. It seems to me a lot of replies here (not all) are a little abstract. I understand the apprehension when it comes to picking a small handful of techniques to represent such a complex and diverse craft. However, I do think it's an interesting question the weatherwitch has. I have been working in motion professionally for a little under a year, and have been an avid AE user for years prior to that. I am going to provide the techniques I find most essential. I think this would be a great thread if artists of all abilities could pick a top five. The answers would surely be quite diverse, or not, but it would be interesting to see. I am working from this definition:
a practical method or art applied to some particular task <LI>proficiency: skillfulness in the command of fundamentals deriving from practice and familiarity; "practice greatly improves proficiency"
1. Camera control. I believe command over the camera is a mark of a great artist. A print designer, for example, has to consider composition and layout once. With a 3d camera, you have to consider composition and layout on an almost frame-by-frame basis, all while keeping the movement as fluid as possible (or not, if/when properly motivated.)
2. Compositing. Most animations are built from many elements. Some combination of video, photos, vector art, text, textures, lights, etc. A convincing or at least aesthetically consistent animation requires adequate compositing skills.
3. Rotoscoping. The quality of a rotoscope can be the difference between a solid piece and a distracting mess.
4. Typography. Understanding the motivations for and appropriate times for different fonts, weights, leading, kerning, layouts, etc. is essential to communicating a message with an animation.
5. Managing workflow. Understanding how different applications work with each other. When to use Illustrator instead of built in AE tools, optimal export settings for the different NLE's, a solid understanding of Photoshop. All absolutely essential.