The horror stories you hear about Art directors usually entail very early mornings and very long nights or even staying over at work. Requiring vast amounts of commitment and tight deadlines to get games on the shelves. Sadly it seems this is the case, but that's what it takes when you consider the amount of work needed to complete a project. It must take a lot of talent to be able to keep a clear head and overview of what the finished game will turn out like and be able to direct every department and direction the game goes in. Its not hard to see why games end up being delayed. Especially with how gigantic the process is. There's departments for Coding, sound fx, music, game assets, art direction and style, concepts art and animation. All overseen by one job title. Its no wonder that the large list of requirements I have seen for jobs applications are so huge. Not just in the game industry but some are expected to have experience in TV and film.
I would also think they would need to have a very strong, traditional art background. As they would need to have a keen eye for lighting, colour, perspective, scale, and composition to allow precise judgement over all the work produced by the team. Its not hard to see why its important as these skills apply to all art styles produced for games.