Gold for riding training. Travel time is a SIGNIFICANT time sink while using quests to level. You don't want to wait to get it. In fact, while questing, the second you ding 40/60/70/80 you'll want to hearth and train it IMMEDIATELY! 10304 gold is the base cost, which will be modified by reputation. Also, make sure you're in a guild with the 10% riding speed bonus unless you're a class that already gets a mount-speed buff.
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.

Excess Potion of Accelerated Learning: Acquired in your Garrison (part of the Warlords of Draenor expansion), this gives you a 20-percent bonus to experience from killing monsters and finishing quests. It lasts one hour, but you can buy it multiple times and use it back-to-back. Alliance players buy it from Sergeant Crowler, horde players buy it from Sergeant Grimjaw. Does not work above level 99.
5% was admittedly an exaggeration, just felt that low. I'd say it was probably closer to the 30-40% range. It was Artifacts of the Blacksilt, spent a good 10-15 minutes killing Blacksilt Seers to get two damn idols to drop. A quest that had me killing Elder Brown Bears for bear flanks also took quite a while, though that was less due to drop rate and more due to how spread out they were.
Quests from the Tortollans, a race of turtle people, can be mind-numbingly dumb and slow. One of the standards that you'll see is one of those matching games arranged in a grid where you can only turn over two items at once to see if they match. Another, and the worst of all the quests I've done in 13 years with World of Warcraft, is a turtle "maze" where you slowly guide a slow-ass turtle through a slow-ass grid for seemingly no reason other than to get a reward at the end.

I had multiple people ask me why I didn’t do blood or prot for leveling on DK/Pally and the god honest truth is the fact that I know how to play ret and unholy quite well and the limits of the class. I tried leveling as prot and good Christ it felt like I did no damage. If you take selfless healer and wake of ashes you should have 0 issues with leveling as Ret and it is significantly faster. As for Unholy if you take grip of the dead to kite a little, deathpact for healing, and keep up with your Dark Succor (spelling?) procs you will have no issues. I died once as DK and none as pally. The only classes I struggled with a little was WW and Shaman. WW because the aoe felt under-tuned at the time and shaman because they are trash imho.
Fifth: World Quest are back and are still important for leveling your Heart of Azeroth and gaining rep. I do recommend you do these in Warmode for the additional bonuses but if you don’t like PvP I would at least suggest doing a clear of all the quest that don’t give AP in Normal mode then going back and clearing the ones that do give AP in Warmode for the bonus. Emissaries are back and work the same way though it is very important to hold off on any emissary quest that gives you gear until you are at least 310 item level. Once you hit 310 item level, the reward jumps to 340 loot instead of 325? (its somewhere around that area). Try to delay these the best you can, but don’t risk missing out on free loot.
Skinning is a good money maker as throughout your travels you will fight many beasts. It also won't take much time, only 1-5 seconds after every beast. Miners and herbalists have to travel all the way to the nodes. Skinning also provides the passive bonus Master of Anatomy, increasing critical strike (which is especially helpful for agility-based characters who take it with Leatherworking).
In the end leveling quickly in World of Warcraft is a lot about experience and leveling you 2nd or 3rd character will always be quicker than leveling your first one, no matter what leveling guide you use. We at FMG hope you find our World of Warcraft leveling guide useful and if you need help with anything or have comments about the guide, leave us a comment here and we’ll get back to you!
You’ll need things produced by many of these professions to optimize your character, but you only get two professions you can do yourself. Unless you want to grind on a second character to collect all the reputation-gated recipes for other professions, you will probably need to buy the stuff you can’t make for yourself on the auction house. And if the price of the stuff you need exceeds whatever your questing and your followers are bringing in, you might want to sell some of your own trade goods or raw materials in order to finance your purchases.

These two expansions are virtually identical in terms of exp/h. I personally favor Mount Hyjal, and think it's very slightly faster. But if you know Jade Forest well you can just as easily keep up there. Ultimately you'll end up doing both anyway, so it's mostly academic. Every time I've tested Valley of Four Winds or Deepholme, or any other zone in MOP/CATA's level range, they were worse exp/h. You should not even need to complete both zones to ding 91, so it's more important to just pick one and go.
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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.

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