Traditional taijiquan was designed by Taoists to include patterns that set up very specific patterns of subtle energy movement within the body and concentrate energy in specific places. That's why it includes so many asymmetrical movements, and so many movements that have to be modified in significant ways to use in a martial context -- it's the preparatory training for a great deal of Taoist energy meditation. That's what makes it problematic for those who are doing a different kind of training.

Tai Chi Chih, by contrast, is designed to produce a general flow of vital force through the body, not concentrating it anywhere. The movements are simple and repetitive, and you do them on both sides, so the practice is symmetrical. Perhaps the best way to describe the difference is that Tai Chi Chih opens up the channels and lets the vital force flow through them however it wishes, while taijiquan pumps the vital force here and there under pressure.