Lower global cooldown for tanking classes would be a nice touch as it makes the game feel more dynamic and less restrictive when it comes to holding aggro and doing combos. With the long gcd you really don't have any options what to do when tanking because the global forces you to use those AOE threat abilities like overpower and flash extensively in order to keep aggro. It gets very boring quickly when you know you could do it without them if it only weren't for the gcd. Default lower global could possibly draw players to try out tanking classes since they're "different".