I like the idea, just that I would let anyone be able to play any job, but having to level it.

For example:
As said before, when you hit lv. 30 with Gladiator, you can choose Paladin or Dark Knight.
I choose Paladin and I level up Pld till lv. 50 (or whatever the cap will be by then). I would have my unique abilities for it.
If I want to level up Drk, then I will have to go to the guild, and do the quest to unlock Drk.
Then I may be able to go to the guild (or mog house, or whatever system it may be implemented) and "Change job" (something like FFXI) to Drk. The I would be able to level up Drk too.

I like the idea, but I don't like the idea to "If I level up Pld, then I just quit my abilities, and put Drk abilities, and I'll have a full leveled Drk too"

A problem I see, is that SE started this game for casual players but... usually people that plays MMORPG, are not "that casual" This game is SO MUCH casual, I think... : /