I actually have a similar question as the OP but in regards to after finishing the main story and doing all the 'leveled' quests in said area, on the road to getting to 50.

What about leveling up your other classes? Here's what I mean....

I personally started out as a THM, figuring I'd go BLM but I wanted to try out all the classes and after getting around 25, I started working a bit on CNJ. Well, I happen to like CNJ better and now I'm 37 CNJ/WHM. But going back to get some other classes up to at least 15, I found I have no quests other then my class quests (level 5, 10, etc.). There is nothing inbetween as I've done them all already....only reason I was able to get my CNJ out from level 15 to my THM's level was the quicker duty ques and repetitive dungeon crawls untill I got to where my THM was. Then I was able to continue with the story and the area quests it lead to.

continued...