I also think moving will cancel spell casting is not really good idea. The game first concept is to freely cast spell while moving so player will have fast gameplay.

However I think moving while casting should have negative effect too. May be by putting small bar under cast timer. This bar will be full at first but when your character moving or get hit the bar will be shrink. Think of it as Spell HP. If this Spell HP reach zero before cast is finish then the spell will be cancel. If player finish cast the spell before Spell HP reach zero then the effect of the spell will be as portion of what Spell HP left.

for example.
1. Player cast Fire and with no intrruption it deal 300 dmg to an enemy.
2. Player cast Fire while runaway from an enemy. Spell HP reduce to half when finish casting. Fire will deal 150 dmg to an enemy.
3. Player cast Fire while fighting an enemy. That player get hit so he try to runaway. This will reduce Spell HP very fast which most of the case Spell HP will be reach zero before it finish casting. This way spell will be cancel.

This way mage will still be the same image as what they plan. Better to keep distance from an enemy while fighting. It might be harder for DoW if they want to keep using cure while solo but somehow I fell it should be that way. By block / dodge / parry player might lose less Spell HP too so cast in Active Mode will have more chance to success than Passive Mode. And it will be great if player can choose to activate Auto Attack or stop it. It could be first skill for every class called 'Attack' so no one will hit anything whenever they don't want to anymore. Like after put some monster to sleep or when fighting enemy with Shock Spikes on.

I'm hope they would considered this option. It's not ground breaking but this should make player feel like they have more freedom.