Leveling a druid is initially very slow, and is one of the least rewarding classes before level 20, but like most classes, really picks up once you have your full range of talents and abilities to call upon. The main difference with a druid is that while most classes have most of their abilities by their late teens, druids have to wait for many signature abilities until level 20, 30 or even 40.
However, somewhere after 3.1 druids got buffed in the lower levels with a significant mana performance improvement. This enables druids to cast spells without stopping to drink after every fight. This can help significantly with both damage and healing spells. Leveling can be a lot faster than it was in the past.
At this time you have very few options and your method of killing things will be Wrath and Moonfire, finishing enemies off with your staff if you run out of mana. Make sure you carry some water to drink because your staff is painfully slow.
At level 10 you'll receive a quest to get your Bear Form. This is very valuable and you should do it as soon as possible. Your spells are still slightly higher damage than your bear form melee, but your bear form reduces the damage you take significantly and does a lot more damage than your staff. Remember to keep back enough mana to shift to bear form if you run low while killing something.
At level 16 you'll get a nice bonus in the form of Travel Form.
Taking balance talents will give a more immediate bonus because you're doing most of your damage with spells right now, but I'd recommend investing them in the feral tree by choosing Ferocity, Savage Fury and Feral Instinct. The reason for choosing the feral tree will become apparent at level 20.
At level 20 you'll be able to train Cat Form. Cat form is the way to level until level 40, as its damage is at least as good as your spells', while not requiring you to drink between fights. As an added bonus, you can take the Feral Swiftness talent to increase your movement speed by 30%, which will speed up getting between mobs without needing to spend mana shifting to travel form and back to cat. Open with Shred, then Rake and Claw the enemy to death. Rip is useful on tough enemies, but most will die before it can run its course. Use your healing spells to heal yourself between fights as needed, and by the time you need to heal up again you'll probably have regenerated to a full mana bar.
Try to find a Glyph of Claw for your glyph slot.
As a feral druid you'll want to collect gear with agility, strength, attack power and stamina on it. While spirit and intellect will improve your mana regeneration, they should not be necessary if all you use it for is healing between fights.
At level 20 you can train riding and buy a mount. This is certainly worth doing, but isn't actually that much faster than travel form, so if you're short on cash it won't hurt to put off the purchase for a while.
More cat form leveling. You'll gain more abilities in the form of Pounce, Ravage and Ferocious Bite. Ravage is just a better Shred that can only be used from stealth. Pounce is the opener of choice after you get it, because it does a little damage, stuns the target so it can't hit you for a few seconds and allows you to get behind it and Shred for more damage than Claw. Combined with Brutal Impact and Shredding Attacks you can Shred twice before the target recovers and turns to face you. Ferocious Bite is a better finishing move than Rip for enemies that don't live very long (which hopefully is most of them). Talents of choice are Feral Charge, Brutal Impact, Shredding Attacks and then the excellent Heart of the Wild.
At this point you have a choice. You can continue to level as feral and keep tearing things apart as a cat, or you can respec in to the balance tree and take the Moonkin Form talent. Moonkin form grants high armour, bonus critical strike chance and most importantly mana regeneration when you critically hit with spells, making it very competitive as a leveling form. You also gain the wonderful Innervate, which makes mana much less of a problem.
If you continue to level as feral, take Leader of the Pack and Improved Leader of the Pack to supplement your health and mana regeneration, then take Survival of the Fittest. The next talents up to level 50 are a bit lackluster, but Infected Wounds is a decent choice to slow down enemies that run away at low health.
If you switch to Balance then take a build like this, and then take Dreamstate, Improved Moonkin Form and Wrath of Cenarius. Your leveling style will be similar to that you used pre-20, using Entangling Roots and nuking with Starfire. With Celestial Focus and your high armour from Moonkin form you'll be able to stand and nuke enemies even in melee range if rooting is too much effort. As a balance druid you'll want to completely change your gear set to pieces with intellect, spellpower, spirit and stamina. Remember to change your glyphs too.
At level 40 you can train the next level of riding and buy a faster mount. This is highly recommended because the improvement over travel form is just too good to pass up now.
AoE leveling with balanceEdit
Around level 45, I decided to try out leveling as balance so I could AoE things down. I highly recommend it both for fun and the speed at which you can grind. Go fully into the balance tree and move toward Gale Winds asap. Max out your spellpower and intellect as much as possible, and cheap elixirs/chants are probably recommended as well. Moonkins can be fragile so here's how this should work:
1) Cast regrowth on yourself 2) Mount up on your rider 3) Quickly ride to gather 4-5 mobs 4) Use rejuvenation to dismount (that'll put 2 HoTs on you for a good ten seconds) 5) Shapeshift to Moonkin 6) Cast hurricane onto yourself 7) Repeat hurricane until dead
Optional: barkskin is nice to mitigate a little damage. Be sure to use your herbalism heal if you have that profession (recommended for leveling anyway). Use innervate when up to replenish mana.
Notes: You're going to burn through mana like crazy, so get in the habit of carrying around an 80 stack of water. I did this at 45 and owned with it, so I suspect it is viable much earlier. Once you hit 50 and get the second rank of hurricane you'll be an amazing AoE killer. Have fun!
At level 50 feral druids get a major damage boost in the form of Mangle. Mangle completely replaces Claw, and in addition it increases bleed damage and damage done by Shred (and Maul) to the target. Take Mangle, Improved Mangle and King of the Jungle. King of the Jungle makes Tiger's Fury grant energy when used, which is particularly useful to maximise pounce stuns. By level 56 Pounce, Mangle, Shred, Tiger's Fury, Shred, Shred is possible in the 4-second pounce stun, which will leave most enemies nearly dead before they can even hit you.
Balance druids will gain equally exciting Typhoon and Force of Nature talents, giving them a knock back for enemies that get close, and a potent cooldown for when they get in trouble (which is actually quite easy with Typhoon because it has a deceptively long range). Take Typhoon and Force of Nature, then finish Wrath of Cenarius and take Earth and Moon. Eclipse is very powerful when standing and nuking instance bosses but is difficult to use properly while leveling, so avoid it for now.
At level 58 move to Outland immediately because the gear there is significantly better than anything in the old world.
At level 60 you get possibly the best benefit to being a druid - free flying! Train Flight Form at your trainer and you'll get the next level of riding for free, saving a tidy 650 gold.
After getting their level 60 talent, feral druids will want to invest in the restoration tree for Naturalist, and Balance druids will want to fill out the balance tree some more, then work towards Intensity. In either case, you're now an experienced enough druid to make your own choices from here.
What if I want to heal?Edit
The most important thing is to collect a spell-casting gear set (which balance druids will already have). Until level 40 you should find that while wearing a decent set of caster gear your innate healing abilities are sufficient to heal instances without talents. After level 40 you may find it difficult, and may want to use a hybrid feral/resto or slightly modified balance build. Taking restoration talents up to Intensity and Omen of Clarity, including Naturalist, will give you some healing benefits while helping your feral damage as well. The balance tree has some talents that help both damage and healing, such as Lunar Guidance and Dreamstate, and investing in these talents instead of pure damage talents can help you to heal.
If you plan on leveling resto, make sure to get all the +Spell Power gear you can. Most SP will buff both healing and damage, and you will do most of your damage in caster form, from range. If needed, swap to Cat Form once you obtain it for melee fights. Elites are relatively easy to take down in this spec, since you can swap out of Cat form to put some heavy heals on yourself, then swap back to cat for the melee DPS. Once you reach level 50 and obtain Tree of Life Form, Try to stay in instance leveling as much as possible, since that is what Resto is all about. If you plan on being Resto, obtaining Dual Talent Specialization can be helpful, especially if points are put into the Feral Tree to make leveling easier at that point.
What if I want to tank?Edit
A feral damage build is just fine for tanking right up until level 80. Just follow the feral path and you'll be able to tank just fine. Collecting high stamina pieces will help. If you have trouble then drop some damage talents for Protector of the Pack.