There are a lot of games where you're paying a fixed price or at least a comparatively minor contribution that isn't necessarily tied to the level of enjoyment or participation you get from it. In these cases, you're expending the monetary equivalent of activation energy and then the game is basically whatever you make of it.
At the other end of the spectrum are games where you have to make an ongoing investment to get something out of the game that matches this investment with a linear factor or worse. Online Poker would probably be an example, but it lacks the social pressure exerted by some MMOs, games where you actually feel you're going to a kind of workplace and where real money is at stake constantly (which in turn is a large part of the players' motivation to achieve things).
Diablo III made me feel this way, for instance, it's more a free market simulation than an action RPG. Eve would be in that category, too. Many other MMOs either discourage trading for money and/or are making time and effort the primary player currency. Contrast this with games where real money is closely coupled with any kind of high-level content.
