I was wondering when this'd get brought up. I've hinted about this in the past in a couple interviews... Yes, there was a very significant underlying change here, that may have implications for theorycrafting (though minor).
I don't want to get too deep into the under-the-hood workings of WoW servers, but here's a super short version. Any action that one unit takes on another different unit used to be processed in batches every 400ms. Some very attentive people may have noticed that healing yourself would give you the health instantly (minus client/server latency), whereas healing another unit would incur a delay of between 0ms and 400ms (again, on top of client/server latency). Same with damaging, applying auras, interrupting, knocking back, etc.
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."
We no longer batch them up like that. We just do it as fast as we can, which usually amounts to between 1ms and 10ms later. It took a considerable amount of work to get it working that way, but we're very pleased with the results so far; the game feels noticeably more responsive.
I can't guarantee that you'll never ever again run into cases where Guardian Spirit went on cooldown and the tank still died... but it'll be literally 40x rarer than before, and the whole game will feel more responsive too.