Originally Posted by
Gadix
50% haste will make your global cooldowns exactly 1 second, which is their hard limit. Stacking more haste will not give you a lower GCD. It used to be that haste beyond this was useless, but now it's only a soft cap. Haste beyond 50% will still make VT, DP, and MF do more damage, but you won't see as much increase as before 50%.
As the other poster mentioned, it's almost impossible to get the soft cap passively, but with raid buffs + speed potion + heroism/bloodlust you can easily go beyond it for a few seconds. This also means that sweet spots aren't as important now, since any amount of haste will give you more dot damage.