Tanaka's departure is due in part to his health; he suffers from a "major illness" of some kind, according to Andriasang's translation of a Famitsu interview. Tanaka also cited a desire to work as a "single creator" on future development projects as a reason for resigning his post as Final Fantasy XI's producer.
Aside from FFXI, Tanaka's portfolio at Square dates back to 1984's The Death Trap, although his work on the original Final Fantasy probably garnered more exposure. He also worked on Final Fantasy II, III and IV, as well as Xenogears and Chrono Cross. Akihiko Matsui, currently part of the Final Fantasy XIV team, was named as Tanaka's successor. Matsui will work on both FFXI and FFXIV, primarily focusing on FFXIV until Version 2.0 is completed.