IN-COMBAT REZ: Main tank drops dead mid-battle? Druids have the only in-combat rez of all the classes. Not only does it bring the target back alive, but it restores most of their health and mana as well as avoids any kind of resurrection sickness.
CROWD CONTROL: Druids can use Cyclone to help break up pulls and can also put mob classified as Beast or Dragonkin to sleep for 40 seconds with Hibernate. Each spell can only affect one mob at a time, but can be very useful to divide and conquer many encounters.
WHAT NOT TO EXPECT FROM A RESTORATION DRUID
OUT OF COMBAT REZ: If you are not the main tank who has dropped dead in the middle of battle, don't expect a rez. Yes, druid rez works in combat, but it's on a 30 min cooldown and is designed for emergency use. "I don't want to run back from the graveyard" is not an emergency. So no attitude if the druid refuses to rez you because you're too lazy to run back on your own. Druids need their rez to save the group when things go bad, not summon you back to the party so you can go AFK to TiVo Battlestar Galactica.
DRUID DE-AGGRO: Druids don't have many effective ways to shed aggro. Healing themselves while being beat on is mostly counterproductive since it just keeps them on top of the hate list (unless its necessary to keep themselves alive.) At L70 Druids get Cyclone, a crowd control spell similar to Warlock's banish, but works on all mob types. The problem is that Cyclone may stop the mob from pounding on the druid for 6 secs, but also makes the mob invulnerable so there is no way for the tank to regain aggro until it wears off, leaving the druid in the same boat when he cast the spell. Druids also have a de-aggro ability in cat form, but the time it takes to transform and then use the ability and transform back is time the druid isn't able to heal himself or others. If you see you druid healer in cat form, things are very bad.
Fortunately, they have their 20% damage reduction spell that runs for 15 seconds. But the tank needs to be actively regaining aggro if the mage or warlock is unavailable for crowd control. Remember: dead druids don't heal.
TREE HEALERS: These are often raid healers who have taken the 41 point Restoration talent that transforms them into a tree. Tree shape isn't necessary for trash mobs or even the first boss. But it sure doesn't hurt on the tricky 12 mob pulls or the final boss in a heroic. The downside to Tree form is no straight heals can be cast, only heal over time spells and a movement reduction on the druid. But most Tree druids know how to compensate for these disadvantages already.
HYBRID HEALERS: This is a more common talent build for Druid healers. Investing a significant portion of their talent points into the Balance (Moonkin) tree allows them to solo well while still being effective 5 man healers. Don't be surprised if they toss in the occasional Moonfire into a fight early in the crawl. As long as they don't get aggro they can speed up the rate mobs die and that's always welcome.
FERAL HEALERS: Druids specced out for bear or cat can often heal 5 mans with the right gear. They will take less chances, but they can do a fine job. Be sure to ask if they are willing as many feral druids aren't interested in healing full time.
David Bowers takes the reins for next week's druid column. In two weeks I will be back with a discussion about looting issues for druids.
[Edit: Took out implication that all healing druids only wear leather. Many of them wear cloth items for the greater +healing and +mp5 as pointed out by Gurei in comment #2.]