1) Why so late? If this had come out a couple of years ago, they could've cemented themselves in Windows 7.
2) AFAIK the real reason for Microsoft not making an app store for native apps is that they can install spyware, browser toolbars, change browser home page, change default search provider, eat up battery, install always running services and what not because they're not sandboxed.
How will the Steam store deal with the inevitable barrage of such user hostile apps? Will the approval process be highly exclusionary or inclusionary? Where are the app store terms? What's the cut? 30%?
I think dealing with adware, spyware, malware and grayware is going to be a challenge. If they're pretty exclusionary, expect devs to get pissed off.
3) Sadly Steam Store won't be allowed to include WinRT apps but Microsoft allows links to desktop apps on their app store.
If they solve the malware problem, I think they'll have a huge market and help devs immensely.
Anyone know or remember Intel AppUp Center? It completely failed to take off.