"I would say my role on DeathSpank (and everyone else's, really) was to create something lots of people would want to pay $15 dollars for. The difference being, the way I see to do that is to help make the most engaging, amusing game possible. As it's a discrete product, once we've transacted, we're done. I don't need to find a way to get another $15 (and another, and another) out of you. If we've really done our job right, you'll like the game so much, you'll tell your friends about it.
"Social games monetize doing the opposite. You want to create a game that's compulsive and then start erecting barriers. Then you provide incentives to pay to remove those barriers. Players who aren't whales are only valuable because they are good bait for whales. They can spread the game around, attracting whales and they provide social pressure to keep playing the game, keeping the whales around and paying. But they're not valuable at all (in fact, they're only a cost) unless they're attracting or keeping whales around. David Hayward summarizes this as a 'Fuck the users' approach and I'd say he's about right. Naturally, this collapses to making the games into rote, simple actions, as to attract and retain as many people as possible. Social games don't have to be high quality beyond a certain minimum threshold. Quality in something free is actually an expense that offers diminishing returns pretty quickly."
-Hothead Games Programmer Nels Anderson discusses his theory as to why free games tend to be worse than games you have to pay for.