Literally just do quests as efficiently as you can. The difference between picking a perfect route and just doing whatever is like an hour, tops. I've already leveled 7 characters in bets because I won't be playing at launch (baby due in aug) and I've been under 12 hours on all but the first. I actually broke 10 on DH, but that's bcuz war mode and DH leech is OP, not because of a special route.
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.
Kael'thas - Alliance Kael'thas - Horde Karazhan - Alliance Karazhan - Horde Kargath - Alliance Kargath - Horde Kazzak - Alliance Kazzak - Horde Kel'Thuzad - Alliance Kel'Thuzad - Horde Khadgar - Alliance Khadgar - Horde Khaz Modan - Alliance Khaz Modan - Horde Khaz'goroth - Alliance Khaz'goroth - Horde Kil'jaeden - Alliance Kil'jaeden - Horde Kilrogg - Alliance Kilrogg - Horde Kirin Tor - Alliance Kirin Tor - Horde Kor'gall - Alliance Kor'gall - Horde Krag'jin - Alliance Krag'jin - Horde Krasus - Alliance Krasus - Horde Kul Tiras - Alliance Kul Tiras - Horde Kult der Verdammten - Al... Kult der Verdammten - Horde
To give you some perspective I leveled both a LF Ret Paladin and BE Rogue (so one started from level 20 and the other from level 1). I focused on getting the achievements I was missing for the Loremaster, so not the most efficient route by any means. I was using full enchanted heirlooms (minus the ring) and the WoW-Pro addon though, as well as having War Mode turned on all the way.
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.
Update: The reason for less experience from gathering is due to how Blizzard revamped professions and broke them down by expansion. In order to get full exp from gathering you'll simply need to find a profession trainer for the relevant expansion. (WoD you just mine or herb the first one ypu come across and the game gives you a scroll for learning WoD gathering).
There are various upsides to this imbalance. Max level players are able to complete lower level content with ease, allowing them to swiftly complete missed quests and achievements in earlier zones, and can travel through zones without being constantly attacked, since mobs will only attack players several levels higher than them if walked directly into. Max level also makes it possible to solo earlier group content such as dungeons and even raids, making farming loot such as mounts far easier, and for those interested in a challenge opening up a new subtype of play at slightly higher levels.
Edit 20 It’s been a week since this post and a lot of people are still reaching out to me with questions which is fine. I have had a few different people reach out to me to help make YouTube videos based off this guide and more info in BFA which I’m more than willing to do. Whoever gets the videos up first I will link them here for you guys to enjoy.