Playing WoW would probably be regarded as a hobby here and any income from it treated as hobby income.  While you can sell WoW gold for real money, the purpose of the game isn't to generate something of real world value.  Bitcoins, on the other hand, are intended to be a currency and there are no non-financial reasons for mining/buying them - whether people mine them or buy them, trade them or hoard them, they are doing so in expectation of a financial return - that's likely to influence how they are treated for tax purposes as opposed to WoW gold, frequent flyer points, or Facebook credits.