So I've been regularly running heroics on all four of my healers for a couple of weeks now, and it strikes me as interesting how different they feel to play at times. It's not so much about the healing spells they use, or even reactive vs. proactive healing (direct heals vs. HoTs and shields), but rather the difference between what I'd call aggressive and defensive healing.
Aggressive healing in this case means that the healer is generally as aware of the mobs as of his own party members, can easily join in to add some dps and may even actively decide to use offensive abilities to make healing easier.
Somewhat to my own surprise, I found the shaman to be the most aggressive healing class. Take bloodlust: While healers obviously also benefit from its haste boost, I find that, in all honesty, I don't give a crap whether it's up or not while I'm healing a five-man, not from a purely selfish, how-fast-are-my-heals point of view anyway. It's a dps ability really. And yet, even for a resto shaman it goes without saying that my group will expect me to use it at an appropiate moment. I actively had to train myself to not forget to use it at least on the last boss of each instance, and I still often struggle to gauge correctly at which point earlier in the run it's "safe" to use it so it will be available again by the time we get to the end. Also, at the end of the day it is useful to me as a healer even if don't care about buffing myself, because if the boss dies more quickly due to the boosted dps, I have less work to do.
Then there are the totems. I continue to be surprised by how many groups I end up with that don't have a single caster in them, rendering my well-meant flametongue totem completely useless. In such cases I often end up going with searing or magma totem, even if they only do piddly dps, simply because I can drop four totems with one click, and if one of them has no other use than to dps - even though I'm the healer - I guess then I'm doing some dps! Also, my fire elemental totem comes out at least once per dungeon regardless of other casters in the group, by the same logic as bloodlust: it's added dps at no cost to my healing, and the boss dies faster.
Then there are the interrupts. They are generally something that's supposed to be covered by the dps, but I've found that even in my better pugs, hardly anyone seems to interrupt anything, like, ever (unless they want to try for Less-Rabi). One boss where I've felt this particularly painfully and frequenty as of late is the Devourer of Souls in heroic Forge of Souls. He spams an ability called phantom blast which does huge damage even to a well-geared tank. It can be interrupted, and there's even an achievement for interrupting all of his casts, but does anyone ever actually do it? Nope.
And here I am with my shaman healer, and a ranged interrupt on a short cooldown. It doesn't take a genius to figure out that it's a lot more efficient to simply interrupt as many phantom blasts as possible on my own instead of constantly having to spam the tank back up to full health. It requires more general awareness and as I say, what I consider a more aggressive way of playing a healer, but it works.
The second most aggressive healer in my opinion is the paladin, largely because of their judgements. A good holy paladin will want to keep up the haste buff from judgements of the pure one hundred percent of the time, and for that he has to judge and thus attack a mob at least once per minute. I can't think of another healer who has to attack something as part of their healing rotation. Also, even while the haste buff is up, judging a mob with light can help a lot with healing in heavy AoE damage situations for example.
Also: hammer of justice. Stunning mobs is once again something that's generally considered the job of dps and tanks, but I find it a powerful defensive tool... which kind of goes against me saying that paladins are aggressive, but the point is that when another healer has to defend themselves, they can mostly just heal themselves and cry for help. A paladin can actually knock the mob out on their own (and then hope that someone will come to help within the next six seconds). It also works well to protect others, such as when you see a skirmisher go for a squishy clothie in heroic Azjol-Nerub. Again, this is something that requires you to look at what's going on with the mobs around you.
Priests are pretty defensive healers already. They can happily spend a whole instance just looking at their party frames and ignoring the mobs, because it's not like they can do a lot about incoming attacks. Fearing mobs away is an option but a risky one, not to mention that a lot of mobs in Northrend instances are undead and thus immune to fear anyway. Shackle works as crowd control against undead, but like similar effects it's hard to pull off effectively even if you want to, what with all the AoE flying around these days. About the most offensive thing a holy priest can do is spam holy nova in AoE situations, which does some damage as well as healing the party. It's also pretty easy to add a bit of dps as a holy priest with dots and smites, but by then you've already crossed the line from aggressively protecting your party to just dpsing.
Druids are the most defensive healers in my opinion (in PvE anyway), simply because of tree form, which prevents them from using any sort of aggressive move at all. Sure, you can choose to stay out of tree, so you can also cast hurricane, wrath and moonfire while nobody is taking damage, but you actually have to take a penalty to your healing to do so, so it's definitely not something that it's encouraged by the class design.
I still like all four healing classes, but I have to say that in five-mans, the added utility of the shaman's and paladin's more aggressive moves to prevent damage to the party makes things a lot more interesting. I often hear complaints that healing is boring (which I don't agree with, but that's beside the point) and that Blizzard should make it more interactive, for example by people getting healed from healers doing damage. This concept already exists and it's called a shadow priest's vampiric embrace, it's repeatedly been nerfed to the ground for a reason and is now nothing more than a small gimmick for what's still very much a dps role, so trying to turn every healer into a shadow priest doesn't stike me as a good idea. I do wonder however, if giving healers more aggressive, non-damage abilities that force them to interact with their environment more could be a way to go.