The role of an art director is to co-ordinate the efforts of the various artists working on a project. Even if the game does not have a theme as such the designs still have to be consistent in order to fit into the game.
I suppose the managerial responsibility of the director limits their creative output as it is more of a overseeing and guiding of the other artists who produce the actual artwork. I would have thought the art direction in games would be similar to films with the evaluating of the same aspects for example characters or scenery.
An art director who I hugely admire is Hayao Miyazaki as his films are just breathtaking in terms of visuals and story line. I think all Studio Ghibli films are pretty damn good though my favourite has to be Spirited Away, where Miyazaki did such an awesome job as an art director expressing his feelings and life experiences within the movie. Also the way each scene has been hand-painted just gives so much more depth and truly shows the skills of the artists.
It seems that to become an art director you would start as an artist first and work your way up- as to help guide the artists in an effective way you would need to be able to relate and have the knowledge and experience to deal with the problems they encounter.
See ya ~Emi ^o^