Finished DQ4 so I'm playing through this now.
I have to say I'm a sucker for open class systems so I'm enjoying this so far. Each character is in one of three predetermined class trees: fighter, scout, wizard. Each base class can specialize in attack, defense, or magic which opens up two subclasses each. So for example, if you configure your fighter for attack, you open up Bushido and Master Samurai. So a total of seven classes for each archetype.
You can change classes any time, as long as you can meet the requirements for the new class (which are like accessories that monsters drop). Each class gains experience as you use it, and can gain up to three levels. Each level permanently unlocks a skill for you. You can never change your base, so your main character is always in the fighter tree for example.
Anyway, enjoying the class system and the plot so far. I actually didn't see the mid-game plot twist coming, which is rare.