The problem with face paint is that, from the ones I've seen, it flakes off during the match, which means that for a long match, by the end of the match the "mystery" is gone (if they used the same type of paint that circus clowns or rock groups like KISS used, it might stay on better). As someone who basically sees wrestling as a live-action comic book, the masks help the superhero image (and it's still possible to show emotion depending on the mask, look at Rey Mysterio Jr for example).