Druids are the keepers, which locked in slumber for generations in the world. They awoke to meet the threat of the Burning Legion during its recent invasion. After Archimonde's defeat, the Druids chose to remain in the waking world and help to rebuild their shattered lands. The Legion's attack left a terrible scar on the natural order, and the Druids seek to heal it.
The Druid gives players several play style options. In normal form, a Druid is a caster that can fight with spells or weapons. In Bear form, the Druid becomes a Warrior with Rage. While in Cat form, the Druid becomes a Rogue with Energy and stealth. The Druid can also transform into two other special animal forms. With its ability to heal itself and fellow characters, the Druid can take on the role of a Priest. A Druid is not as versatile in its abilities as a Priest is, lacking the spells Power Word: Shield and Resurrection, but is otherwise a very capable healer.
We are mainly talking about the healing side of a Druid. A good Druid is one that knows the advantages of his class, and also is aware (and admits) the weaknesses within his class. The advantage of Druids is how versatile they are, they can do many things at the same time, they aren't the best, but they are some kind of Jack-of-all-Trades.
On a healing point of view, as a Druid, there are advantages and disadvantages. The details are as follows:
Over time, healings are the best of any other classes. These would be Reiuvenation, an instant cast spell and Rogrowth, a spell that crits often with talents.
The only Combat Rezzing spell, called Rebirth. After one of the latest patches (prolly 1.10), it can be used with Nature's Swiftness, and make it an instant spell.
Innervate, that lets you regenerate mana at a faster rate, and can be used on the druid or on any other target (like a Priest).
We use Leather items, which provide more armor than Cloth items (compared to priests).
Barkskin is a spell that avoids spell interruption caused by damage.
The ability of being able to shapeshift, as the situation fits. The group wiped, there are respawns, what's better than stealthing and being able to rez?
Mark of the Wild / Gift of the Wild is, according to Blizzard, one of the most powerful buffs in the game.
We don't have any kind of spell that can reduce our aggro, at least not like a priest that has Fade on the form of caster.
Compared to a Priest, our spells take too long. With talents, our best spell can take up to three seconds while Regrowth takes 2 seconds.
Regrowth is a mana killer and takes too much mana and doesn't heal as much as a Healing Touch.
Our Area of Effect Healing Spell doesn't work in the same way as a Priest; ours is channeled, theirs is instant.
Depending on the items, usually druids have a lower mana pool than a priest, and usually we run out of mana faster.
As a healer, there're two kinds of things that can benefit Druids: +healing and +mana/5seconds. There have been many discussions which one is better and it may depend on your healing style. Let's explain it a bit. Before continuing, keep in mind that having just a bit of these won't help at all. Also, both of them end up in the same, because both are used for something called Mana Conserving, which will be explained later on.
The advantage of having + Healing is that it lets you use lower rank healing spells, in that way, you are able to heal for the same as using the high ranked ones, but still, using less mana. This stat also affects your Healing-over-Time spells, which are Rejuvenation and Regrowth, which is pretty awesome. + Healing is also mostly useful on short fights, in which you can sit regen.
The advantage of having + Mana per 5 seconds is that it lets you regenerate, regardless of if you are not casting or casting (works different as Spirit). Think about it as having a permanent Blessing of Wisdom. The main point of this is being able to regenerate mana as fast as possible, to be able to cast. + Mana per 5 seconds is mostly useful on long fights, in which you need to regenerate fast to be able to heal.
On the hand, Druids don't have a huge mana pool as a Priest can, so we have to use things that use last longer on fights. Regardless of what you choose, you need to keep stacking the stats, because having a little bit of them won't work at all. Items that have that stat have low INT but it doesn't matter if you are mana conserving.
On the other side, as a Druid healer, it's not recommended at all focusing on items that only provide tons of INT, because you won't be efficient at all, and because you will burn your mana faster than someone that has +healing or +mana/5s. Items like %crit aren't as helpful as the ones explained before as well, because, with talents, Regrowth already has a high chance of critting, and, well, Healing Touch heals for a lot, and a constant stat is more beneficial than a something that has a chance.