I think the headline of the article is pretty misleading as he means that Windows 8 is a catastrophe for Steam because of the Windows 8 app store. Looks like everyone's upvoting the article because they seem to think that Gabe meant that Windows 8 will be a failure. Instead of having to go through Steam's distribution, games will have the option of going directly to the Windows 8 app store and get featured there, not…
"Microsoft is charging between 20 to 30%" - I think that's too much, it must be much less than that, but I don't know for sure.
Store fees are calculated on a per-transaction basis as well. These fees are based at 30% of the app price, until your app reaches $25,000 or equivalent of lifetime sales (aggregated across app sales and in-app purchases) when they change to 20% of the app price.