The Off-GCD abilities not casting right when you press them is something that has bothered me tremendously since I started playing the game. If the button is lit up, pressing it should trigger the ability immediately, not ignore my input and fail silently. Having lit buttons like that just makes it so that the UI is lying to the players, and because of that makes the game feel sluggish and unresponsive. Having buttons that ignore input is horrible UI design and just leads to me mashing the button as fast as I can when I want to use the ability.