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!
Most of the time you spend leveling will involve questing and slaying monsters, but there’s also a lot of travel. Travel earns you little experience (you do gain a bit for discovering new areas), so it’s wise to keep travel to a minimum. The new level scaling system, which scales zones to your level within a preset range, helps with that. You can choose what zones you want to experience and stick with them until you finish their quests.
While we don’t know what will happen to players that choose to side with Sylvanas when she’s ultimately toppled from power, it seems that their actions will make some kind of difference on the game — consequences that could mortally harm one of the troll’s heading up Saurfang’s rebellion, Zekhan (introduced in the “Old Soldier” cinematic, he won the community’s heart and has been rechristened “Zappy Boi”), who is primed for Sylvanas’ crosshairs after opposing her.
I like his recommendations 2 level 1-20 zones to get to 20, then do all the 2nd zones since they are designed for walking (Horde: Northern Barrens, Silverpine Forest, Ghostlands, Azshara, & Hillsbrad Foothills if you need a little more. Alliance: Westfall, Loch Modan, Darkshore, Bloodmyst Isle, and if you need a little more Redridge Mtns, or Duskwood). Don't even bother with the higher level areas, too spread out. Finishing up a Monk now and I had done all these zones before the update, the running between quest and the 50% experience cap is driving me crazy.
A "stat squish" and "item squish" was implemented to lower the numbers used in the game, e.g. a legendary item previously with a level of 1000 reduced to 265. Unique class-specific buffs are added back, i.e. mages' Arcane Brilliance and priests' Mark of Fortitude. Titanforging—a random event that raises the initial item level of an item gained via drops or rewards—is still in the game; however, if the item is one of those affected by the Heart of Azeroth, then that item cannot be titanforged.
Void elf - "Many have sought to harness the corruptive magic of the Void. Most who tried have fallen into madness. Determined to use this power for the good of Azeroth, Alleria Windrunner is the first mortal to succeed at defying the shadow's whispers. Coming to the aid of a group of her kin who nearly gave in to the darkness, Alleria has vowed to train these Void Elves to control the shadows within them and pledge their newfound powers to the Alliance."
Spoiler: A lot has changed. It can feel daunting to jump into a huge world of which you know very little, and World of Warcraft is gigantic. You will feel overwhelmed if you just rush in. As a returning player who used to write for the freakin’ World of Warcraft magazine (RIP), even I find it hard to figure out exactly where to get started adventuring if I haven’t played the game for a while.
While it’s clear that Blizzard is eager not to spoil the future of Battle for Azeroth, it seems pretty unlikely that players who want to be pure evil will ever get their day to shine in World of Warcraft. Despite the decision being added into the game, the good guys will likely win and the threats to Azeroth will continue to roll on by, getting knocked out by players one at a time.
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.