This is how I'm 99 percent sure the job system will work based on the interviews and everything I've read:

You level up a Gladiator to 30 and a Marauder to 15. This fulfills the requirements to unlock the Paladin quest. After completing this quest you receive a "job stone" that you can equip to have your level 30 Gladiator go into "Paladin Mode". This "mode" gives you access to unique skills that you gained through your job quests and blocks you out from using certain abilities from other classes that don't fit the theme of this mode, "tank" for paladin.

[Assumption]Going into Paladin mode changes your class stats as well. Your strength might drop a little, but you will see corresponding increases in vitality and maybe mind.

You gain access to special paladin only armor and weapons that further enhance your specialty as a tank.

Every 5 levels or so 35, 40, 45, 50, etc. you hit a threshold where a new paladin quest becomes available. Completing the quest ranks up your paladin and grants you some new abilities.

This all implies that Paladin won't be a stand alone class it's dependent on a base armory class that is temporarily transforming into that "mode" for party play. So if you want Paladins and other jobs to be a separate entity, all I can say is sorry that's not what they're going to do.