I think a lot of the replay value, and possibly the entertainment value overall, is going to come from how they distinguish between the classes. It's more than just elements, at least - it seems like the classes have more of a cultural implication, and there will be different quest lines for each class. If the spells amount to more than just elemental attacks, then there's room for a lot of variety in the combat, too. I'm hoping for something like Avatar: The Last Airbender, minus the actual ongoing conflict between the Fire Nation and the rest of the world. (Although that conflict might itself be interesting, but more likely material for a sequel in this case.)