Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.
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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).
Heirloom experience should be boosted, now that zones scale. If a zone scales along with me, it shouldn't matter if it gives me 20 levels instead of 10. I'm still enjoying the experience, even as a veteran player. I don't need to see every zone on every character again; that's the point of heirlooms. I can still enjoy the leveling experience, and the scaling, and the full outdoor experience of the zones -- and still get to the end game content I enjoy in a somewhat reasonable time as an employed adult. And the change wouldn't affect new players.
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.
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.