The biggest complaint I heard from people is that their mask was ill-fitting. It was either too tight and wouldn’t +stay looped around their ears, or it was too loose and practically fell off of their face.
If I wanted to create the best mask, it had to be ergonomic -- form-fitting, but breathable, even wearable for long periods of time. It had to let in fresh, filtered air and be comfortable even in the heat, or when playing sports.