Sweat/Hot/painful masks: yeah, I think on that allot, every face has a different fit, and when theater masks do not fit, it can be distracting and painful. What would be the most comfortable mask material you can recall, (something like felt, or a plastic, leather, paper.....???)
Thanks for responding Weazie.

1. How to "act" without using your face.
I am thinking that by "act" you mean "with out facial flexibility" as the face is still a vital part of mask performance. The difficulty comes when bringing together the character of the mask to the performance in the body.

2. Visibility
Technique and time solves this. I've seen too many shows where the director thought that, "mask is a neat idea" and then directs the show as if it were a typical actors' theater piece. This is assuming that by visibility you mean, "actors using the mask" and not "I can't see the damned thing".

3. Diction/clarity
That's why there is a half mask. Full masks are not supposed to be talked through. A noob mistake.

4. Sweat/hot/painful masks
Leather stops sweat, but who wants to spend $600 minimum on a leather mask that you have to break in. Hot is always a problem in paper and plastic mask. Felt works best for that; to a point.

Acting without the face was a primary focus of the research conducted by such mime theorists as Etienne Decroux and Jacques Lecoq.

The solution is to work towards an articulation of the rest of the body-- especially the torso-- which is neglected in most western theatrical training.