Druid is the one and only hybrid class in World of Warcraft.
When mastered it becomes the most exciting class to play.
Druid can fill any of the rolls in the game (tank, dps and healer).
So the fact is that whenever you get bored with a specific roll, you always can decide to play a different one without rolling a new character.
So having a Druid is a big advantage.
Leveling a Druid is efficient with two of the three talent builds.
Leveling a Druid - Which talent tree is the best for leveling? First of all, do yourself a favor and please don't level as Restoration.
Restoration tree is purely for healing and NOT damage dealing.
So as long as leveling a Druid fast requires killing the enemies (when questing) this build is just out of the option.
Balance talent tree is focused on enhancing Druid's offensive spells.
You become a powerful caster.
This build is also called a Boomkin.
That's because with this talent tree you will be mostly in Moonkin form (boosted up with "Improved Moonkin form").
If you consider to level a Druid with Balance talent tree than you must realize that as all the casters you require mana for casting.
That leads to a little downtime for drinking (mana regenerating).
But this only occurs at low levels.
As you progress in levels this downtime becomes smaller and smaller.
A good side of this build is also that if you like instances, than you can heal without a problem up to level 50 or more.
The ultimate leveling build (and of course the fastest one) is the Feral build.
With Cat form (at level 20+) you become a pure melee killing machine.
Play-style is some kind of similar to Rogue's (stealth, energy, combo points).
With this build you kill your enemies extremely fast (due to high burst damage), you also don't need mana for damage dealing.
So all your mana is spent for healing yourself after some hard encounters.
If you level a Druid with Feral build then you are the fastest leveling class in the game.
Leveling a Druid - Questing? Absolutely! Questing is the fastest way of getting XPs (experience points).
But beware! Tons of quests out there are designed to slow you down.
You get a small amount of XP and you spend a lot of time to complete this kind of quests.