One thing about Microsoft though is that they leverage things like email lists internally to provide many opportunities for participation one might think of as primarily characteristic of these flat organizational models. For example, a lot of the work that was being done on competing with Linux when I was there was taking place on the basis of this sort of flat model.

The point I am making is that these wouldn't have occurred to the Valve founders in a vacuum. They were built on top of what they already knew worked. So it's not a matter of playing around but rather trying to copy what you think works well in your past experience and ditching the rest.