What is Gabe Newell's definition of open source? With Ubuntu (which, let's be honest, is the closest to a consumer-friendly distro there is right now) you got a limited repository and practically no backwards compatibility, and the only viable way (other than apt-get) to install new software is via the built in Software Center, an app for downloading apps á la Windows Store.
Not to mention that any Linux OS is going to have a very limited library of games seeing as more existing titles are made for DirectX which isn't compatible with that platform.
In my experience, trying to install and enjoy games and other software is so closed and restricted it's claustrophobic, and it's not only because of the major Linux/NT kernel incompatibility.
While Linux might be more open in theory, I assume that most people's experience with Linux is going to come in the form of a pre-packaged, pre-installed Steam-booter locked to big-picture mode, which is by no definition open, particularly not compared to Windows.
Gabe's been talking a lot about open platforms, but he has yet to address how this open platform his company aims to provide would actually, in practice, enable users any more freedom than what you got now with Windows.