The very nature of the class system, and what the developers wanted was that any single character can play all classes. I don't think they would suddenly start restricting players now as far as picking Black mage or White mage. I think the 'Job' system will be more like a title you attach to your character, or a subclass. 30CON Spec: WHM
For example: I'm a rank 30 Conjurer, I now have the option of taking on a quest that will allow me to become the job "White Mage". Once said quest is complete I can choose to take on the title of White Mage, or stay a Conjurer both would be rank 30, but Conjurer would maintain its spells like Fire. White Mage would gain a passive +20% healing and buff time, in addition to gaining some unique skills and spells, but it would become unable to cast offensive spells (like fire). I can then take on the "Black Mage" quest, which would unlock that job. I now have 2 jobs that are based on a single class. I can choose to be White mage or Black mage, but I can't be both at the same time. I can also opt to stay conjurer and be a mix, without gaining any special skills or spells. Obviously you would have to be out of combat, and possibly at an aetheryte crystal to change.
This is the way I think it will turn out. Players maintain their ability to do everything on a single character. I guess the easiest way to explain would to put it in WoW terms.
You make a character and choose priest. As you level you begin to specialize, Damage or Healing. Shadow priest vs. Holy priest. I think we will see similar effects in the new system. You will still be a conjurer, but you will fit into a more defined category. Similar to WoW you can re-specialize in town, or at a select NPC.
I really can't see a better way to make the system, but I guess we can all wait and see what these guys whip up for us.
Yes I know there are more than 2 specs for priest in WoW. I'm leaving out Discipline because it is essentially another healing role for priest. At one time it was more of a PvP spec, which doesn't apply to FFXIV anyway.