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.
My second least-favorite part: leveling ranges. Zones are divided into leveling ranges; some are for levels 10-60, others for levels 58-80, and so on. On the surface, that appears to make sense; while there are odd moments where you're presented with the content from two expansions at once, it does mean that you're more or less going through content in order, at least by expansion if not by level.
Excess Potion of Accelerated Learning: Acquired in your Garrison (part of the Warlords of Draenor expansion), this gives you a 20-percent bonus to experience from killing monsters and finishing quests. It lasts one hour, but you can buy it multiple times and use it back-to-back. Alliance players buy it from Sergeant Crowler, horde players buy it from Sergeant Grimjaw. Does not work above level 99.
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If you have a lot of gold, you can use it to pay other players to do some of the game’s less exciting activities, like fishing, herbing and mining. Some high-end guilds like to amass large stashes of gold so that they can provide consumable crafted items like stat-enhancing alchemy flasks and cooked food items that buff players, as well as enchantments and gems, to their raiders.