The solution, I believe, comes from the DK talent Purgatory. I love healing a DK with Purgatory because I know I have time to plan my moves. Even when he's at 1 health I know I have at least a three second Purgatory window before he dies. That gives me time for a big heal, or a trinket, or a cooldown like Void Shift, or two GCDs of instants.
I think we should make Purgatory the standard mechanic for all players. Call it "Mortally Wounded" and give the same period and cooldown. Now healers can start thinking about how they want to heal. There's time to cast that big slow heal even if the tank is at 25%. There's time to spam some efficient heals to top people off, knowing if something changes drastically you have time to change to flash heals. You can wait until "Mortally Wounded" goes off to use that Lay on Hands or Void Shift to make sure you get the most out of it.
The Purgatory ability does have a cooldown, which I think works so that it doesn't become a permanent crutch for every situation, and it makes healing more interesting for me because I tend to shift from healing to absorbs when a DK has the purgatory debuff to give myself some extra breathing room.
The latest interviews seem to indicate that the developers are giving up on the "efficient" heal that people stop using completely after they get out of questing greens. I propose the Purgatory option should be explored instead of turning the game of healing into a choice between "big" and "fast" alone. I'd like to be rewarded for knowing how to triage properly. I just don't want to get blamed because my big heal gets to the server 2 miliseconds after the server decides the tank died.