What hard/technical skills are most important for Opera Singers?

For singers, obviously, vocal technique and vocal hygiene are a must. If a singer gets sick or is unable to sing for a portion of a rehearsal process or gig, they will likely be replaced and put on a "do not hire again" list. Knowing how to care for your body and voice is absolutely crucial. Aside from vocal hygiene, singers must know their languages and the variations in pronunciation between languages or even from a spoken language to its sung counterpart. Directors shouldn't have to coach a singer on how to properly pronounce words in whatever language the opera is in. The point of the director is to create a cohesive and impactful artistic experience, and that can only be done if the singer is singing the language correctly and understand what they and the other artists around them are saying.