Substance

The form must adhere to the tai chi classics.
Without a comprehensive understanding of the three treatise, you cannot determine whether or not you are performing the sequence properly.

Underpinning the classics is the need for substance; internal strength/power.
Whole-body movement.
Neigong.
The ability to move your body as fluid, fully-connected network of parts is essential.

An extensive understanding of jing is also necessary.