> So when will Steam let me add third party repositories for software?
This was never a comparison between Steam and the Windows 8 app store. The two are not equivalent, because one is baked into the OS, while the other is 3rd party software.
What is comparable is the Windows 8 app store and the Ubuntu Software Center. And it's very clear that the latter is much more open.
Once it's been released? There are various closed source software projects that currently allow you to install software through the Software Center, such as VirtualBox and Chrome. You just add click on a link on their site to add the PPA to Ubuntu's package management system.
This is all FUD being spun up by Valve because they don't like competition for their app store.