My current idea of the job system is this:

*You will not be able to use cross-class abilities (totally defeats the purpose of the job.) And restricted to the abilities set forth by the class and the job.
*You will gain access to more unique abilities through quests to learn more abilities that better suits the job and its role as you level.
*Class Rank and Job Level are the same. If you're a 50 LNC, you're also a 50 DRG.
*Classes will still be usable in situations and jobs won't totally make them obsolete. (Melee won't be able to raise unless on a job)

Some things I am not 100% about is the prerequisite jobs. It HAS been mentioned that Paladin is made from a level 30 Gladiator and a level 15 MRD. Perhaps, this means that Paladin will use abilities from MRD and GLA.

I also am not sure if the new jobs will require completely different weapons. I'm quite sure they do. I do not see a Bard having access to Ifrit's Bow, as it needs a bow harp. And I don't see MNK having access to Ifrit's Claws as it requires some different type of fists (Dragon theme, animal theme, etc). I could be wrong tho^^