Most heirlooms only work in a limited level range in their default form and must be upgraded to work at higher levels. That costs gold, and you’ll need around 30,000 gold to outfit yourself with a fully upgraded set (counting only those that give you bonus experience). You may need to spend some time on a boosted character to acquire the gold you need to outfit another character with heirlooms, but it’s well worth the effort.
101+ At this point you'll start taking a penalty on quest/bonus objective experience for every level past 100. Treasures, however, remain worth their full value all the way through level 105, at which point they'll be worth something like 15% less, and then another 15% more when you ding 106. I recommend that you stop questing/gathering entirely, and collect all the treasures you can stomach, only avoiding any annoying to reach ones inside caves or behind elites/rares(where scaling does bizarro things).
Zygor's Leveling and Loremaster guide is the ultimate tool for players who want to level up quickly and earn the coveted Loremaster achievement. This guide will lead you quest by quest through every zone, telling you exactly where you need to go, how to complete objectives, and everything else required to reach level 120 in the fastest time possible.
Also note that power leveling is sometimes used to refer to paying someone else to level your character to cap as fast as possible (i.e. they power level for you); a distinction used by Blizzard is to refer to the paid leveling as "power leveling service." This behavior is against Blizzard's Terms of Service. Power leveling yourself, which this article covers, is perfectly acceptable.
I dont know if this exist already, if it does I couldnt find one, but it would be nice to have a guide with the location of all daily quest broken down by Alliance, Horde, Faction, Neutral, etc. I know there can be variable quest for things like Fishing and Cooking but just having the location for those quest givers would be great. If it were a stand alone addon that would be even better!
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.