http://www.newyorker.com/online/blogs/elements/2013/04/brain...
I think one of the things that makes SC2 special is not simply the gameplay, but the stress that it puts you under at the same time. Not only do you have to make the correct decision, you have to make the correct decision NOW and if you don't, you will suffer consequences. This combined with the need for precise control forces you to either develop or suffer repeated failure. As it's also - in it's highest difficulty - a competitive game, there's also social pressure to not fail. Look only to these comments to see that; one commenter states his rank, others try to discredit his position. This pecking order is something important to them.
A game like the Sims as a control, is very different. You tell a story, you might have game elements, but there's much less stress, you have much fewer decisions you need to make in a short period, and if you make an error it changes the story rather than typical cause disastrous failure. You can not develop and still enjoy yourself. The fun is in the narrative, not "win" or "lose". There is no competition.