Reaffirming what Iscah said about Job quests: the level 30-60 and the level 70 quests are especially important as you only unlock new Job abilities by doing those quests (60-69 and 71-80 they're learned automatically). And as your base class stops learning abilities by 50, it's pretty obvious your effectiveness in battle would be severely hampered otherwise. So definetely keep up to date with your Job quests.