Yeah, level 15 is right where you get held up if you do absolutely everything aside from Leves in a new area it seems.
There are quests for lv15+ on all the areas pertaining the 3 regions tho. Will those quests be enough to level you up all the way to 50? No. But there are still quests, even if there are many areas without quests in the beta, it's already been said that all areas will have their corresponding level quests.
And I mentioned lv15 for a particular reason: Dungeons. As soon as you reach that level you are set levelling wise, and can go all the way to 50 by "grinding" them. Yes, it will be a grind because you'll have to go through some of them more than once, but a good thing will be that you'll gear up that class by doing so.
Now, will there be quests to help level more than 3 classes (at least before a big update -2.1- that might introduce new quests), I don't think so, but then again, if you have taken more than 3 classes to cap or near it, you'll be more than familiar on how the game works and getting lv15 on a new class just on the Hunting Log, FATEs or leves, won't be an issue at all.