What happens to the urge
In MMOs in particular the games are designed to make us do stuff, sometimes for a long time, to achieve some virtual goal or item. A Ysharros writes in her blog even if you are not really particularly interested in a certain achivement or activity, the game environment manages for us virtual mice to go for that virtual cheese, somewhere in the maze – and we get some satisfaction out of it.
In my work life I sometimes have to spend a fair amount of time out of the country, such as now when I am pretty much only in my home country during weekends. Being an MMO enthusiast, one thought would be that in those cases the gaming sessions when I am home would be more, longer etc to make up for that time. But instead I find that it is the other way around.
In these periods I am less inclined to run after the virtual cheese and I find gaming sessions can be even shorter or not as fun as when I can play more regularly, even though I may do the same type of activities in the games.
So I think that much the gameplay in MMOs work because they manage to condition us to play them regularly – without that there less urge to do the various activities presented to us. At least as many are created now. Being more casual friendly in the MMO world I think currently means that one need to spend less time each day, but the activities often still require some regularity to give us a virtual boost.