GamesIndustry International - Valve is the champion PC gaming deserves.
Valve's intention with these moves is not modest, nor is it in any way disguised. Valve wants to own PC gaming - to the same extent that Sony owns PlayStation gaming or Microsoft owns Xbox gaming, if not in the same manner. Valve wants Steam to be a third pole of the core gaming market (fourth, if you count Nintendo, although they're really playing a different sport entirely) - one which appeals to a somewhat different market and operates on very different principles to the traditional consoles, but which is focused around the Steam platform for distribution and functionality just as much as consoles are focused on PlayStation Network or Xbox Live.

The Tech Report - Are Valve and AMD about to ruin PC gaming?
This could all make PC gaming somewhat daunting to newcomers. Today, buying a decently powerful PC opens up access to a huge library of games, from point-and-click adventure titles to the latest cross-platform shooters. Next year, things will be different. Because of SteamOS, not all gaming PCs will be able to run all PC games, unless one is prepared to install a second operating system. And because of Mantle, buying a machine with a GPU from the wrong vendor could mean missing out on critical optimizations.