Correct, GCD shields don't stack. That's most of it. I believe damage buffs/debuffs from the same job also don't stack; two Chain Strategies won't have any additional effect if applied at the same time. It's also important to note that mitigation is multiplicative, not additive. This means that all mitigation after the first will have diminishing returns if stacked, so blowing all of your defensives on one attack won't just put abilities on cooldown, but they'll also reduce less damage than they normally would if you space them out.