I've been meaning to respond to this.
It is quite cunning how game developers have quarried the vast fields of neuroscience and psychology to make games more addictive for the rest of us. I mean, for anyone who's ever mapped for a sp game, you know in order to make something what we call "entertaining" is just the right balance of action and exploration. I think most might agree that good SP mapping is correctly applying "variable ratio rewards" in just the right regulation.
With L4D and multiplayer games in general (excluding MMOs) we find players supporting other players in just the same way. The game developers have already done the leg work to set up a kind of dichotomy between the team (or every-man-for-them self) matches where the core gameplay mechanics reinforce a variable reward ratio situation. Especially in L4D(2) where I think Valve really has struck a chord on the mixing of SP and MP where you can get your rewards from other players AND be able to control it as part of the core gameplay too. With the inclusion of the "Director" Valve still gets the control they need to keep the game interesting. I mean they've talked about it themselves about how the game "watches" your team's stats and adjusts itself to keep you "ENTERTAINED".
Perhaps we should be more weary to this; more-so than I thought initially necessitated. It's like gambling without loosing. In the Skinner box, what does the rat have to lose as it presses that lever? There is no negative reinforcement. With smoking, there's that fear of degrading your lungs to the point of cancer, but with games, other than your time, I don't see the long-term negative reinforcement. I'm not saying there should be one, I never want to stop playing games; but that can be a serious issue for some people.
I don't subscribe for a minute to the thought of developers proactively including impediment mechanisms to limit players access to their bought games,

But does being aware, or more aware of this now affect your gaming habits? I think it's nice how Steam can report the number of hours you spend in any (steam registered) game, it kind of helps to regulate myself sometimes, -if you care about those kinds of things.
But after saying all that, I still encourage developers to keep investigating the neurosciences and psychological methods to make their games more addicting. Because for me, more addicting DOES in-fact mean more fun, within the context of video games anyway. We could talk about what fun is, but I think that would warrant an entirely new conversation anyway.
What do you guy's think?
Blog:
www.playingarchitecture.net
LinkedIn:
Eric Lancon
Twitter:
@Riven202