Runes of Magic shone a light on the problems that trading real money currency could cause the economy. Diamonds (the ROM pay for currency) rapidly became the currency of choice for AH traders, which caused hyperinflation of the gold to diamond exchange rate. Initially it was ~5k gold per diamond. By the time I quit a month later, it was about 70k gold to one diamond. Everything on the AH worthwhile buying was listed in gems. Gold farmers prices were way higher than the cost of buying diamonds for those inclined.
Note: Euro prices seemed to have settled down to 25-35k gold per diamond, but US servers are currently at 70-85k. I've found people looking for diamond sellers willing to buy parcel's of 5000-10000 diamonds for 20k per diamond (100-200 million gold).
I don't know how they adapted to this (never bothered to go back and play again) but any system where you have a work for currency and a bought currency which the player can trade, the world currency will eventually devalue if there are no ways to sink all the funding coming in from real money purchases.
Strangely enough, Eve balances this by making things destroyable. Billions of ISK can easily exit the game with the destruction of certain assets, so atrophy acts as a brake for hyperinflation. I doubt GW is going to feature permanent breaking of items etc though.
EQII's system of real money servers, where people could buy items from other players (or plat) also worked well because you could pay your sub with money proceeding from exchange sales. All money bought and sold was created at the pace at which you could farm it rather than introducing a new currency which is only limited by how much your customers want to spend in the real world.