The main problem with it that everything gets greyed out for the GCD, preventing you from seeing which abilities are ready to be used next until the GCD finishes. This makes it difficult to time your next attack, especially for a class with proc/conditional based attacks.
While in combat, everything is greyed out 99% of the time as a player should be using abilities fairly constantly.
If abilities went grey only because they were on CD, not the GCD, it would be fine.