Fully rested experience (this is the blue color your experience bar takes instead of purple) grants 200% more experience per kill. Quest experience does not work the same as mob experience. Instead of just using the rested bonus, quest experience pushes the bar further upwards extending the rested bonus. Say if half your experience bar is blue. If you turn in 10 quests all at one the normal experience from them all could boost a character a full level. Unlike mobs where the experience gain will use up said bonus - the quests merely 'push' the bar further. Doing this could net you a level and still have the 200% bonus for mob kills applying to milk the rest bonus.

The community was apparently frustrated with Blizzard for a lack of communication concerning the issue, but it looks like that was due to the fact that their heads were down as they worked to understand the problem. It was explained that they didn't want to just roll out a quick bandage, as that would likely cause unforeseen issues down the road. Instead, they're crunching the numbers and digging into the code in the hopes of discovering a legitimate source of the problem.

Drustvar is far and away the hardest Alliance zone, which is exactly why you want to do it first. If you have legendary items, they’ll still be usable, and if you don’t, this zone will still be easier early on than it will be later. Drustvar is also extremely fast and will let you jump out ahead of your competition in other zones, which means fewer players around to take your quest objectives.
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Guild Banners. The Banner of Cooperation and Battle Standard of Coordination will increase the experience from kills. They both have a 10 minute cooldown, but when you use one, the other is only locked out for 2 minutes. I was very bad about using these, so it might be worth using a macro to drop them on your first attack against an enemy. Keep in mind you must have reputation with your guild before being able to buy these, so new character may have a bit of a wait before picking them up. 

Professions have changed dramatically in Legion. While before you simply had to pay for recipes when you were in town, now the trainers as you to gather certain materials or defeat certain targets in order to unlock additional recipes. How can you profit from this new system? Well, to compensate players with the harder recipe unlocks, the developers have implemented a recipe rank system.
As always, I will monitor and update this guide as new information comes in. If I've gotten anything glaringly and completely wrong, just tell me with as much data as possible. The idea is to find the fastest possible leveling method, NOT to argue about how good or bad individuals are. If you have hard data, post it! If you have an idea, let the thread know so we can test it!
Many people have asked whether we recommend leveling as Enhancement or Elemental. There’s no definitive answer here; it depends entirely on your comfort, your gear options, and if you’re in a group or solo. Enhancement benefits more from Honor Talents in open world leveling, and Elemental has stronger AOE damage for mass pulling. Feel free to experiment with both specs.
I just decided that I would simply finish leveling my hunter the hard way, but when i can get heirlooms ill make a warrior and follow your guide to 80. I definitly want an 80 warrior for cataclysm... seeing as worgen warrior is going to be a crazy tank! By the way, is using heirlooms with your guides ok? And what class should I be for a tank? I will obviously be switching to worgen anyway but i would prefer best tanking until then. Thanks Jame!
As you continue to push through end-game content and you raise your reputations with various groups, you unlock more quests that send you to the opposing faction's continent where you generally try to mess with them, gather intel on their movements, and, of course, kill a whole bunch of them. These quest lines are fun, but generally pretty short and don't actually propel you through much of the three opposing zones.
By constantly doing quests, you will experience a very different game than if you just level through pure grinding. You'll be out there making a contribution towards the Alliance or the Horde. Doing these quests takes you through the heart of the game's story AND provides the fastest rate of experience. Have fun in World of Warcraft and enjoy developing your hero - your adventures will gain you great reputation amongst the land.
As you're leveling, I generally recommend you just follow the natural flow of the progression through each zone. They're laid out fairly reasonably. But don't be afraid to hop around, especially if you're near a quest objective. You shouldn't actually need ALL of the zones listed to reach 62. Pick the ones you like best. It's much more important to simply be as focused as possible and flow from one quest to the next.