If you love running old content for transmog items, you'll love the new legacy loot system. Whenever a player enters a dungeon or raid where they are 10 or more levels above the content, the dungeon or raid bosses drop an amount of loot equivalent to what would have dropped for a full group when the content was current. That means players could earn loot for five people in dungeons and loot for 20 people after soloing a raid. The legacy loot system also drops other gear types, like cloth dropping for a plate wearer. It's now much easier to get the transmog gear you want, and you can also collect gear for your alts on the same run.
So much new stuff. The war questline expands. Island expeditions get a couple of new locations to explore. Azerite armor gets tweaked so it’s slightly less stupid than it already is. While we wait for the servers to go back up, Blizzard’s prepared a handy survival guide for Tides of Vengeance. I suggest looking it over, or else risk not surviving at all.
Anyway both got to 110 at around 50h played. I messed up with my Pal by going to Legion once I hit 100 instead of finishing treasures and bonuses in WoD. All in all, if I picked the best zones and was only trying to level as fast as I could, I would probably could have done it in roughly 40h. Note that it would have a been A LOT faster with a Monk as both Ret and Rogues don't really have a decent AoE until 40+ (45 for Ret and 63 for Rogue I believe). The daily also helps a lot. I know that leveling my Monk before the pre-patch was insanely fast. Might be slower with the changes to FoF, however.
Bags: This SHOULD be a no-brainer. The cheap-o version are Netherweave bags, but if you have the gold I recommend you get the largest bags you can afford, since it means you won't have to stop to clean them out as often. In fact, if you have an excess of gold, it's probably not even worth your time to stop and loot mobs that aren't quest objectives. This seems like a small thing, but over the course of 100+ levels, all those clicks to loot add up. 

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!

Warmode is one of the real stand-out features of Battle for Azeroth, even if it is a somewhat controversial one. Warmode tries to revolutionize world PvP like the game has never seen. By switching on Warmode, you enter a new version of your server with only other people looking to fight out in the world. The result is a more volatile world where fights can break out at any time, and groups from to go fight the opposing faction.

If you’d rather read than watch, there are also plenty of resources you can use to acclimate yourself to World of Warcraft, figure out what to do next, and ask questions when you’re feeling stuck. For the basics, we recommend reviewing Wowhead’s “Guide on How to Play World of Warcraft,” which is definitely worth scanning if you’re new to massively multiplayer online games or just want to get a sense of how Blizzard’s approach differs from others you’ve played.
The Horde 12-20 (Barrens + Stonetalon Mountains) guide has been rewritten and revamped.  In addition, there has been numerous tweaks to the speedrun route to make things faster and easier to follow.  Also, in case you haven't noticed I am now adding about 40% more info per step and redoing all the images.  Unlike with my previous vanilla guides, I am now giving a brief explanation of where to go and what to do with each step along with any other helpful tips.
Take breaks. Unless you're straight up no-lifing it over a weekend or something, this leveling process is going to take ~40 hours. But any time you do need to step away, log out. That tiny little bit of rested exp is insignificant...but you never know when something might come up and you'll have to leave for a longer period of time than you expected. Better to be logged out and safe(especially with War Mode on) than AFK waiting for a d/c. Plus, you'll need to stretch, eat, shower, and maybe sleep. XD

XP Potions: The Elixir of Rapid Mind and Elixir of Ancient Knowledge no longer drop. They can be bought on the AH still but are way to expensive for it to be worth it. The Excess Potion of Accelerated learning is available from your garrison quartermaster but only works for level 91 to 99. As of Patch 8.1 you can also buy the 10% XP Potion Draught of Ten Lands.

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