Abilities can't be used till animation ends.
I recently stopped subbing because quite frankly the game feels "unresponsive" compared to other MMORPGs I've played.
In FFXIV when I use an ability and the GCD (Global Cool-down) is activated, I cannot use an ability that is off the global cooldown until the animation for the ability I just used finishes. The same is true for animations that last longer than the GCD, I cannot use another ability till both the GCD AND my current animation have finished playing. This prevents me from using a move that's on the GCD and off the GCD at the same time (ie: interrupt + use an ability for rotation). Every other MMORPG I played "effects" (ie: damage, healing, etc.) happened as soon as an ability was used, not after the animation had finished. While FFXIV's combat looks fabulous, this "locking" of the input while animations are playing feels clunky. I am hoping SE switches more abilities (if not all) to do their affects as the ability is used instead of waiting for the animation to end.