Why is the comparison being made just between Steam and Microsoft's app store? MS hasn't yet launched their service and what it's uptake will be is yet to be known. Alternately, we have the Mac App Store. It's been around for a while, has a huge uptake, and many developers already have data on how successful (or not) they've been. While Macs are only ~20% of the desktop/laptop market, it's a very lucrative segment.
I would like to see a comparison with how Valve will compete or cooperate with an existing, proven desktop marketplace rather than speculation on something that hasn't launched yet.
I would like to see a comparison with how Valve will compete or cooperate with an existing, proven desktop marketplace rather than speculation on something that hasn't launched yet.