Global Cooldown.

[Claire_Pendragon]

Back in the early steps of 2.0, the developers released the Beta, and its the only real hint we have towards why they decided what they did.

The GCD was 3.5s instead of 2.5s
They were debating on 4.0, 4.5, and 5.0 seconds between attacks.

They mentioned needing to "Quicken" the animations of attacks, in order to fit their animation within just 3.5 seconds.
A lot of attacks that carried over from 1.x were just too long and flashy.

They also had attacks that could take nearly 5 seconds, and back when you pressed the button, the enemy would die, or take dmg. Which makes things weird, to see an enemy fall over dead, then see your character build up power, start their dance, and a big explosion, all on the corpse of an enemy.

So players complained the dmg and effects need to trigger AFTER the animation (Hello benediction/Hallowed Ground) to avoid this constant awkwardness (it really was awkward to be honest)

American Players complained 3.5s was just way too slow, and boring.
Japanese Players complained it was just too fast, as useing chat was impossible between attacks, like they were used to in FFXI for on the fly strategy changes.

So THey felt the GCD was almost a perfect in the middle.
Just long enough to type somewhat, but just quick enough not to bore the NA players.

Eventually they said they would lower the GCD to 3s instead, due to constant complaints.

So they made Weaponskills have a 2.5 GCD and spells have a 3.5 GCD. (PLD spamming flash was soooo slow, you always lost hate by the time u could use flash again)

At some point, i think 2.1 or 2.2 they changed it so spells only took 2.5 to finish the GCD, and yet again, at another later date (no idea when, maybe in the 3.1+ patches) they made most spells finish casting in 2s, rather than 2.5, helping with things like BLM procs, and using oGCDs before the GCD was ready. (Minimizing clipping)

The current system is something thats meant to please as many people as possible, while actually pleasing no one.

But to be honest, I grew to like it, and it has created some gameplay you cant really find in other MMOs, such as the over complicated rotations for some DPS. (Not that I like the rotations, just saying it is different) but for the GCD< i can like it, as its easier on my hands for more than 30mins of play. Any other MMO, my carpal tunnel kicks in too bad (Which i got from 1.5s GCD MMOs needing a button pressed so darn much.)

EDIT: (Forgot to mention i play every MMO with a controller, regardless if it was intended or not, and when it comes to FFXIV, only targeting is nice, the actual way they handle modifier buttons, etc, is actually worse than most. I find WoW easier to use the controller than FFXIV. And playing a class with a GCD of 1s, and a means to lower it, I found no issue using a controller to activate abilities with a controller. (Only issue being tab targetting sucks in all games outside of FFXIV and FFXI)