Rosanne N. answered 09/24/21
Professional ASL Interpreter & Educator - Bridging Communication
Facial expression would be the most important feature. Grammar is in facial expressions as well; you can say a lot without using any hand signs! :-) Without facial expression, meaning can be lost, yet without hand signs, meaning can still be there as long as you use proper facial expression. Basically, all the non-manual, which is body movement, facial expression and mouth morphemes (the shape and movement of your mouth).
Facial expression is hard for some hearing adults to grasp when learning the language. So, it's best to practice in front of a mirror so that you master facial expression. ;-)