Well unlike combat system or quest systems etc, a job system is more a mental problem then a physical programing problem.

Very little relative program code need to be changed depending on how they approach it. It's been more of a "get it right", then anything.

Though "getting it right" haven't been FF14's forte so that might be why it's taking so long.

On a very basic level, a job in MMO is nothing more then a selective grouping of skills and stats.