Looking outside MMO-land, here is a good example of how an economic model can undermine a game. Excerpt from the GameSpy review of Gotham City Impostors:
The real catch though is how long it takes to earn these unlocks. Want to jump in and create your own Boy Wonder or Jokerette? You need to level to unlock any customizations, then unlock the slots, then get keys to unlock individual items, then level more to get their mods and level more to create extra loadouts… endlessly. Unlocks aren’t inherently a bad thing, but Impostors takes it too damn far — especially when it’s selling a $3 “XP booster” as DLC.
Clothing yourself is an even bigger nuisance…
The game advertises “1000 levels of player advancement overflowing with upgrades and unlocks” which is how you get the game’s other advertisement of “More customization than you can shake a shotgun at.” That becomes encouragement just to buy the unlocks rather than waiting for 1000 of them, plus more costume bits to buy with tokens, plus “premium” items.