Skinning is both highly profitable and convenient, in that you will be skinning the monsters that you're already killing as you level. A skinner starts by collecting relatively worthless ruined leather scraps, but soon moves on to light leather, which can be sold for a good profit. An excellent way to farm leather is to skin the kills of other players, especially if you are following along in the wake of a group. However, do wait to start skinning until it's clear that the other player has abandoned the kill. Don't assume that the other player doesn't skin just because they are not a leather-wearer, for example. Note that your chances of getting a better grade of leather increases with your experience; it is possible, though rare, to get light leather from rabbits. Higher grades of leather yield higher profits. A skinner/miner has potent earning power through the auction house, but often runs out of inventory space.
KUI Nameplates offer the highest degree of customization and accuracy. For example, you can easily configure the nameplates to enlarge and change color when a target reaches a certain percentage of health, which can be very helpful for classes with executes. They always correctly display your debuffs, and their click box is very accurate. Some setup is required, although it is very simple and straightforward.
If a companion dies during the battle, it will need to be resurrected before it can participate in another encounter or be summoned as a traveling companion. Get that pet a medic! Or in this case, a Stable Master. Stable Master NPCs can heal and revive your pets for a small fee, and there is no cooldown on the Stable Master's heal. You’ll find Stable Masters in major towns and cities across Azeroth.
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.
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.
ElvUI is a complete replacement for the default User Interface. Not only is this a popular AddOn among top players, but we here at Icy Veins use it for all our characters. ElvUI eliminates most of the inefficiencies of the Blizzard interface, which are many and varied. It is extremely user friendly, and takes very little time to set up and master. It also comes already furnished with almost everything you need; unit frames, raid frames, minimap, buffs and debuffs, and much more. You can easily unlock and drag all the components around so your UI will look exactly the way you want it to. ElvUI is clean, easy to use, and a great starting point for your UI.
From “The Burning Crusade” expansion, weapons like Klinge der Zauberei, Die Nachtklinge and recipes for different weapons are really valuable. The best example is Pläne: Teufelsstahlangschwert, worth 500.000+ Gold. These items are World Drops, you can find them in all raids and dungeons. I don’t know any Open World Drops, but that does not mean there are none. Another great transmog source are world bosses and rare spawns on Outland.