Valve is pretty unique in that it was founded by Microsoft millionaires. They had their own personal fortunes to play around with (much like Curt Shilling, except they also had backgrounds in shipping very successful software) while they figured things out.
However, while Steam is certainly a very big deal now I think it is silly not to mention Half-Life as a very important part of Valve's early success. Half-Life was very successful and the fact that it, its sequels and the Source engine begat so many related franchises like Counter-Strike is a big part of the reason Steam was able to leverage a huge built-in audience and succeed in its early days while so many would-be-competitors failed.
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.
However, while Steam is certainly a very big deal now I think it is silly not to mention Half-Life as a very important part of Valve's early success. Half-Life was very successful and the fact that it, its sequels and the Source engine begat so many related franchises like Counter-Strike is a big part of the reason Steam was able to leverage a huge built-in audience and succeed in its early days while so many would-be-competitors failed.