There is an argument among Free-2-Play advocates that "money is equal to time". The argument goes that some players are time-rich and cash-poor, while other players are time-poor and cash-rich. MMO design usually favors the time-rich players. These F2P advocates argue that F2P levels the playing field, allowing cash-rich but time-poor players to use money to make up for their lack of time.
I think that Diablo 3 has shown that this is not correct. Money is not a substitute for Time.
This is because games are emotional experiences. Only, the emotions evoked are not the standard ones, but flow and fiero. The emotions of engaging in an activity, seeking to defeat it, and the payoff from defeating a hard challenge, or finishing a long task.
But these two emotions require time. You have to spend time to get into the flow. And fiero is very often related to how long you spent working on the activity.
When you substitute money for time, there is no flow, and no fiero. There is no emotional attachment to the event, or to the payoff. And that makes the game itself less compelling.