Compared to the once-off package, these include round-the-clock support services, a lack of server queues, four times the amount of characters allotted to each player, increased item storage, guild functionality and other typical MMO offerings. In essence, the monthly fee acts as a gateway to a different business model, netting you an "elite" version of the post-apocalyptic RPG.
The absurdly lucrative business model that powers an MMO like World of Warcraft must be the ultimate temptation and source of envy to those publishing games in the more traditional vein. Whereas most transactions end once the game is purchased, consistent online universes have the player not only buy the game, but continue sending money month after month. Much like slipping on a patch of oil deposited on the driveway by your neighbor's recently discovered geyser, it's not a financial prospect you can easily ignore.