There are lots of changes headed to WoW in patch 6.0, but perhaps the scariest change is the removal of valor and justice points, which have served as a primary form of in-game currency for quite a while. However, Blizzard staff have been working to reassure people that the loss of these currencies isn't the end of the world (or even the end of World of Warcraft).
Almost everything you'd currently buy with valor or justice you'll be able to buy with gold moving forward, so heirlooms will have a gold cost. The one exception is with item upgrades, which you'll be able to purchase with Lesser Charms of Good Fortune (for Mists gear only; Warlords gear won't be upgradable) after patch 6.0 lands. Though the number may change before release, currently the cost is 50 charms per upgrade, with no cap on the amount of charms or how many upgrades you can do a week. Additionally, with valor being gone, the Test of Valor phase of the legendary quest chain is being removed. For those of you trying to level and gear alts before Warlords of Draenor arrives, this will be a big help -- provided patch 6.0's arrival date leaves enough time to make use of them.