The art director is head of the team. They ensure all objects (whether that be a character or a toilet) fit in with the genre. You wouldn't want a shiny brand new loo in a survival horror… think more bloodstained and grimy.
All art needs to portray the genre in all aspects of the game. The lighting, landscapes and objects all aid the atmosphere. The art director insures all of these aspects come together to make the game visually and psychologically appealing.
An art director is ultimately responsible for squishing together various undeveloped ideas into one unified vision. I’m guessing this will mean resolving all the inconsistencies that are created when more than one imagination is used in the formation of an idea.
I get the feeling that they are directors before artists. They may be involved in the mood boards for the game, and choosing which talents will create the art, but in the end, they don't actually do much drawing. But they are certainly creative, it takes a lot of creativity to visualise a finished product and complete a project to a high standard. It also takes a lot of leadership skills.
I can imagine enjoying this job after having years and years of experience wielding a pencil. Perhaps this appeals to those who are a little tired of drawing, but still want to be creative. It seems like a much more mature role, something that involves bucket loads of skill, intelligence and patience… a little intimidating if you ask me…
I bet they miss drawing though.