...I'd like to get your opinion on something if you don't mind. I was recently “informed” that resto druids are “Dead Last” in effectiveness of tank healing. I realize our raid heals are kick ass, but to have someone tell me we suck as tank healers struck a nerve. I’m curious as to what your thoughts are!
Here's the thing: Restoration Druids do not "suck as tank healers." However, we are lacking as compared to other classes in some ways. Any healing class can tank heal, it's just that Discipline Priests and Paladins are the best at it.
Why aren't Restoration Druids just as good?
Well, first of all, we lack OSHI- cooldowns. Nature's Swiftness and Healing Touch/Nourish are effective, per se, but they are not as sustainable as Hand of Sacrifice, Guardian Spirit (Holy Priest only, which are arguably less effective at tank healing as per Juz's comment), or Pain Suppression. You have no prolonged cooldowns, nothing that either improves your effectiveness for a while (such as a Paladin popping Avenging Wrath) or the tank's survivability.
We have no mitigation skills, such as Sacred Shield or Power Word: Shield. Every hit that hits the tank has no extra mitigation besides what our HoTs can tick back up, after the blow has landed. We also cannot heal two tanks at once as effectively as a Paladin can.
Does this mean we are not good tank healers? No. I was on tank healing duty for a Lich King 25 man when we had no Paladin healers, and no wipes came about because of tank death. Restoration Druids are not the best option for tanking, and it is probably a toss up between Shamans, Holy Priests and Druids on who is the less effective one, but by no means are we bad or incapable.
So, how do you tank heal?
The best way to tank heal is to stack all your HoTs. That includes Lifebloom. Now, you have to be a bit more careful about it as Tank healing can be a bit of a mana drain. Manage your Lifebloom stack, refreshing it on OoC procs and allowing it to bloom when the tank could use a chunk of health. Monitor your HoTs and keep them stacked, then Nourish in-between. If there are two tanks, focus on the one you're assigned to and give a couple helper-HoTs to the other when your tank isn't being punched in the face.
So, in summary: yes, you can tank heal. All things being equal, Resto Druids should not be first-choice tank healers, but they are competent and will not fail at the job (mechanically; personally is another issue entirely).