The current penalty for using the one button rotation is an increase to the global cooldown between abilities.
While I understand the logic behind this, I don’t think this is the best route. An increased GCD will impact different specs more or less than others. A haste focused button mashing spec like Fury Warrior will likely be affected more than specs who are slower paced with dots, resource management, or long channels like Dev Evoker.
The GCD increase is also a vague penalty to a newer player who doesn’t fully understand the combat/spell system.
I think that a flat decrease to damage would allow Blizzard to tune the feature better, and provide a more clear downside to using the feature. Seeing the button say something like “With a 25% reduction in damage” is more clear and pointed than saying “With a .3 second increased GCD” and also even for every spec.
Just a random shower thought.