Blue posts have talked a fair bit about the new healing environment they want to create. The basic idea is to give you different spells that you will choose between depending on how the fight is progressing, and to make mana a consideration. I agree with the central direction of this, but there is something about it that didn't make sense to me from day one: a normal heal, a big heal and a fast heal.
Heals vary on two easily quantifiable axes and also on a third, harder to pin down level. The first is healing per second, the second is healing per mana, and the third is the way in which the heal delivers healing. That third one encompasses the difference between direct heals with different cast times, HoTs, Shields, and bizarre things like Prayer of Mending. They are very different, and some are clearly better than others, and that betterness goes beyond and must be measured against HpS and HpM.
But when it comes down to two different heals that function in precisely the same way, there is really only one way they can be meaningfully different. One heal can have higher HpS and lower HpM than the other. Any other variance doesn't work. If Heal A has both higher HpS and higher HpM than Heal B then you don't really have two different heals, or at least things would be in all ways the same for you if you actually did have only Heal A.
And so the normal heal, big heal dynamic works. The big heal is presumably more expensive per point healed. They have the same cast time. You cast the normal heal most of the time and when you think it isn't going to be enough (either because you've been falling behind for a while, you know a big hit will land in a couple of seconds, or whatever) you cast the big heal instead.
But the way that heals deliver healing is a very important consideration. Imagine instead of casting a heal for a specified amount of time you could just channel a heal that continuously drained your mana and increased their health and you'd stop when you felt you'd done enough. This would be the ultimate in giving the ability to quickly react to damage as well as not overheal. I would basically be the limit of making heals faster. On the other end you have a heal that takes 10 seconds, is extremely cheap, and heals the target to full. This is a terrible spell because 10 seconds is too long to go without a heal1.
So a fast heal with the same mana efficiency and healing per second is better than a slow heal. Thus, a fast heal can afford to be a little weaker in terms of efficiency and throughput and still be useful. But how much cheaper can it be? And is the difference between 1.5 seconds and 2.5 seconds enough of a difference to justify a big disparity in effectiveness? Developers were talking about making it so you didn't have to worry about the tank going from full to dead in two swings. If that's really the case, then when am I going to pay more mana to get a heal that lands faster when the slower heal is fast enough?
I was having trouble getting my head around how the normal/fast/big heal system was going to work. They would make reference to the way healing used to work in BC, but we didn't really work on that model. We either worked on the shadow-priests-make-it-so-mana-is-a-joke-and-I-spam-CoH model or the six-ranks-of-heal-and-g.heal-on-my-bar model. We chose between multiple points on the efficiency vs. throughput spectrum much moreso than we chose between fast and slow. These days your tank does die in two hits so you don't dare cast spells that take 1.5 seconds, let alone 2.5 seconds.
Without the breakneck pace for healing, I was having trouble seeing how a fast heal would fit in. It seemed like it would make more sense to give us small/medium/large than fast/medium/large.
Now we've seen the costs and healing amounts of Flash Heal and Greater Heal and it doesn't seem like the developers had any better idea than I did about how to find space for these different heals. Bars show efficiency on the right axis and lines show throughput on the left axis:
There is never really much differentiation between the two heals in terms of efficiency or throughput. Since you will presumably spend a lot of time casting Heal, and switch to these more expensive heals only when you have to, it is hard to believe you will decide that doing 3% more healing is worth the trade-off of being slower. Doing more healing because you aren't keeping up doesn't seem like it will often coincide with wanted to go more slowly.
But I think finding space for both of these heals is much more than a numbers game. Imagine that instead Flash Heal did only about half the throughput of Greater Heal (which is around what I expect Heal to be) and was also less mana efficient. Then we wouldn't cast Flash Heal at all because there would basically never be a situation that warranted throwing your mana away on such a weak heal. You might say, then, that logic suggests there is a medium point where both spells are castable.
That is not necessarily the case. When you are actually healing, you don't get to sit around and look at graphs or run simulations to figure out which heal is the better one to use in the situation. You have to choose very quickly, and you have a limited amount of brain cycles to devote to making the decision. The closer two things are to even, the harder it is to choose between them. Having two different heals that basically do the same thing that are close enough together in effect that it is a subject of much debate which is better means you are probably best off only putting on of those heals on your bar. If you are really smart, you might switch which one depending on the nature of the fight. Choosing on the fly which one to use, however, is putting too much thought into a decision between two very similar spells. Casting either spell now is almost certainly better than casting the "right" one a couple of tenths of a second later.
Bar space is also an issue by itself. I currently have 37 keybinds that I use in combat while I'm healing (though some of them are very situational, I still use them). In the next expansion I'm getting Leap of Faith and Power Word: Barrier (and there is some chance I would have Inner Will on my bars). I don't know how to fit an extra heal-of-a-different-colour into the mix unless it is very noticeably useful. They still have a few months, but I really don't know how they are going to pull off the normal/big/fast trio.
1. In reality this could lead to full-heal rotation setups which would be bad for a completely different reason. But extend the cast time to 20 seconds and you couldn't even do that.