The essence of the art

Every tai chi student in the world should be working towards acquiring these abilities and knowledge:

1. 4 ounces of pressure
2. 5 centres
3. 6 balanced pairs
4. 13 postures
5. Accord
6. Blending
7. Central equilibrium
8. Chin na (seizing): misplacing the bones, dividing the muscles, sealing the breath & cavity press
9. Close-range combat
10. Composure
11. Connection
12. Conservation of energy
13. Double weightedness (avoidance of)
14. Fa jing
15. Filing
16. Folding
17. Freeform triangle
18. Groundpath
19. How to apply every form posture martially
    - at least 7 different ways
    - in a thorough & convincing manner
    - using shuai jiao, chin na & jing
    - in keeping with the Tai Chi Classics
20. Invest in loss
21. Jing
22. Lead into emptiness
23. Listening
24. The meaning and application of the Tai Chi Classics
25. The meaning and application of the martial classics 
    - The Art of War
    - The Book of Five Rings
    - 36 Strategies
26. The meaning and application of the taoist classics
    - The Way and Its Power
    - The Way of Chuang Tzu
    - The Book of Changes
27. Mindfulness
28. Moving from the centre
29. Mutual arising
30. Opening & closing
31. Optimal use of alignment and structure
32. Peng
33. Practical applications of yielding
34. Reeling silk
35. Reverse breathing
36. Shen
37. Sinking & rooting
38. Small circle movement
39. Spontaneity
40. Stickiness
41. Strategy & tactics
42. Substantial & insubstantial
43. Sung
44. Tzu-jan (of itself so)
45. Uniting upper & lower
46. Weaponry
47. Whole-body movement
48. Wu nien (not preparing)
49. Wu wei (not forcing)
50. Yin/yang

Please see Neijiaquan: The Inner School website for an explanation of much of this material.

This list is not comprehensive or exhaustive; it merely serves as an illustration. Many more skills might have been included.
Other tai chi schools might consider additional skills to be equally important.