As the title states, I think offensive cooldowns should be all put on gcd.
Current situation.
Reading these forums, as well as other WoW related forums, shows that a lot of players feel annoyed by a phenomenon called cooldown stacking. Now, this cooldown stacking is plagueing almost every damage dealing class in World of Warcraft, with some classes even ending up stacking 4-5 different offensive cooldowns into a swifty macro (most common examples, but in the end every single DPS has it's own more or less effective swifty macro: Warriors - Berserker Rage, Skull Banner, Recklessness, Deadly Calm, Avatar, strength trinket; Ferals - Incarnation, Nature's Vigil, Berserk, agility trinket).
Why is this happening?
It is quite obvious to anyone that different offensive cooldowns, regardless of how they bolster a classes' ability to do damage, they all share a common trait: they all increase damage. Having more then one cooldown increasing damage means the intended design was for said class to be able to bloster its damage in different key moments during the game by one means or another. The problem with this is that, once again quite obvious to anyone, having a cooldown which increases damage by 20% is much more effectively used when you're already dealing 120% of normal damage then when you're dealing 100% of normal damage. 120% X 120% = 144%. Poping another cooldown which increases damage by yet another 10% over these leads to doing 158,4% of normal damage, and so on. Now, if the intention would have been for the said class to pop the offensive cooldowns separately, then it's not working. If the intention was indeed for damage to increase by 200% when using cooldowns, then there is no point for any class to have 5-6 of them instead of just one, but this is highly unlikely since it's not even resulting in more then one keybind.
How would global cooldown fix this issue?
It's quite simple really: imagine someone starting to use 5 cooldowns one by one, with every single one of them having a global cooldown. This means it would take a grand total of 5 X 1.5 = 7.5 seconds just to start his swifty macro. It would, in turn, give (more then) a chance to the opponent team to react, CC the player, try to los, pop defenses, and so on. The "swifty macro gameplay" would certainly become less effective, shifting the game towards more strategic planning of cooldown usage rather then the current burst fest we are experiencing. Globaling as a strategy would be toned down without a single other nerf being dealt to any of the classes, while emphasys would shift towards putting up longer CC chains on healers, more sustained damage, focusing on interrupts rather then blankets, since stuff wouldn't die as fast in a blanket or even ooming enemy healer.