While this income stream dried up when Legion came out in 2016, Draenor garrisons multiplied the total amount of gold in circulation many times. During the Mists of Pandaria expansion, you were rich if you had a few hundred thousand gold but, after Warlords, rich meant being a multimillionaire. However, the impact of this extra gold was blunted for a while, because WoW Tokens were redeemable only for more game time until February 2017, limiting how much gold rich players were willing to sell.
Special mention is the Darkmoon Faire Top Hat and WHEE! buff. While not always available due to the DMF being inaccessible most of the month, if you happen to be leveling when the DMF is running, it might be worth your time to go and get these. AFAIK they do stack with heirlooms, although I have not personally tested it. Fair warning: These buffs do NOT stack with each other, and the hats disappear after the DMF goes away later in the month(unless you only log in when the faire is running).
Heirlooms. At the bare minimum you make sure all the armor pieces upgraded to 110, since this is the lion's share of your experience increase. Neck and Ring upgrades are secondary, but still important due to being able to enchant them. Trinkets can't be enchanted, but you'll want them at max as well, if only to save time by not having to worry about replacing them as you level.
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