Originally Posted by Blizzard Entertainment
That delay can feel bad just due to the somewhat laggy responsiveness feeling, but also because the state of things can change during that time. For example: Holly the Holy Priest is healing Punky the Brewmaster. Punky spikes low, and Holly hits Guardian Spirit in a panic. The server verifies that Holly is able to cast it, and that Punky is alive (great!). The cast goes off, Guardian Spirit goes on cooldown, and a request is placed for the Guardian Spirit aura (that prevents dying) to be placed on Punky. That request will be filled next time the 400ms timer loops, which happens to be 320ms from now. 250ms later, the boss lands another hit on on Punky. Punky dies. Sadface. Another 70ms goes by, and the Guardian Spirit aura request pops up, and goes "Hey guys, I'm here!... Aww... damn, I missed the party. Sadface."