Also I have seen a few guides on the web that suggest the best way to level fast in World of Warcraft is to grind. I disagree with this as you will always get the same amount of XP for a quest regardless of whether you are in resting time or not so the fastest way is definitely questing.
You'll see that most of the speed levelers use questing more than grinding but you can of course mix the two together and when you are on the way to a destination make sure you do some grinding for best effect.