The problem with "Instant OFF GCD" abilities makes combat feel less rewarding.

[QuantumDelta]

It was my chief criticism in beta, it's slightly amusing that a regen is instant cast yet less time efficient than a Cure III... you can basically only cast a regen on a tank when the tank is at full HP in a hard hitting fight.
There are other examples, and there are DPS examples as well.
The problem with this of course is that if you do let everyone have their instants off cooldown the classes (melee, acn/smn) that have lots of instant cast abilities receive a massive balance related boost, it's not really an option I'd like to contemplate when PvP is considered.
I'm all for reducing the GCD, but I'm not sure about making things instant cast not trigger one.

[RyuujinZERO]

It was my chief criticism in beta, it's slightly amusing that a regen is instant cast yet less time efficient than a Cure III... you can basically only cast a regen on a tank when the tank is at full HP in a hard hitting fight.

Regen is a HoT. Much like a DoT it's all about opportunity cost. While you're spamming cure over and over you're locked into a casting animation and unable to do much else. HoT's and DoT's allow you freedom to take other actions while it's ticking over - if your ward doesn't need a tonne of heavy healing this instant Regen will buy you a grace period before he needs another big heal.

Arcanists are a class entirely built around that principle - DoTing up targets, allowing you to properly analyse the situation and adjust tactics accordingly. So as a white mage you should be familiar with that