2.5s GCD system is a good balance between a fast paced battle system (way faster than FFXI ever was), but still gives you a reasonable time window, and yet it's not to slow either. For example the combat in 1.23 was slower and leans more towards FFXI. Some classes need abit more time to position themselves such as Monk and DRGs that's why a 2.5s GCD is just about right. It's good, not to spammy and not to slow either.