Well the big difference is, as you already said, PoE has fixed classes, Divinity has not.
With fixed classes you have an idea how the classes are supposed to work, how they are different to other classes, how they perform compared to other classes.
With free classes you cant do that. At least not that easy.
For example:
Take PoE and make every skill available for everyone. You WILL end up with a certain amount of skills that are just too good not to use (Like Haste, Adrenaline, Teleportation, etc.).
