Gold. Easy to obtain in small quantities, harder to obtain in large quantities and likely to be used in the purchase of a gold sink item to minimize inflation.
The Burning Crusade saw the introduction of Badges of Justice, which markedly diminished the value of gold. Gold no longer had the ability to buy high-end items. One had to brave the heroics and halls of Kara to gear-advance. However, at the end of Burning Crusade, Primal Nether became salable on the open market, restoring some of the value gold had initially lost. Still, the high-quality items made with P. Nether often required an armor or weapon crafting profession, which again put some restraint upon the ability of people to buy gold for gold sellers or grind gold to arrive at a nice purple set of gear.
Still, it is my impression that gold was very useful for picking up gems and enchant materials. Toward the end of TBC, I power leveled engineering, which cost a small fortune in gold. Gold still certainly is the currency for gaining access to the raw mats made by other professions.
In addition to gear for combat, gold also found its way into a big sink….Artisan Riding Skill- 5, 000g for those of you who may have forgotten. That 5kg got you into a flying mount that was 3.6x as fast as walking and allowed one to bypass “ground” content and for those with gathering skills, to gather up more mats to sell quicker.
In Wrath, is gold going to be as valuable? I can tell you that I may not pick up any additional riding skill unless it allows for a really cool mount. I’m thinking a quartz dragon with fire-breathing capability and a stewardess that does damage to other players (give it a long cooldown for balance). I really don’t see the need to carry other players around in a side-car or, for a druid, on my back. Why I would need to fly faster than I already do is also debatable. I already barely catch some of the nodes spawning on my herb radar due to the speed of Swift Flight Form. Perhaps Blizz will implement something in PvP or PvE that will require the fastest form of flight, and passengers, but until I see the need for that, I don’t plan on sinking my gold into a new riding skill.
That being said, if I have no sink, what will I do with all that extra gold? I am confident that I’ll be able to buy the things I need with gold made from questing, fishing, herbing and cooking. Now that inscription will require herbs and less pots will be required from alchemists due to potion sickness, I think herbs will remain a viable gathering profession. For a casual Druid- herbing remains the best profession due to the ability to gather while remaining in Flight Form.
As for inscriptions, enchants and gems, these items will likely still be in high demand and available on the AH for Good Ole Gold as will the new Frozen Orbs.
In conclusion, I don’t see gold leaving the game as a viable currency. Even without a riding skill gold sink, which I have faith Blizz will create somehow, gold will continue to have some value. Compared to TBC and Old World, I would expect that value to be slightly less.