Microsoft isn't "killing" PC gaming. All games I've tried still run the same on Win 8 as they did on Win 7 including some stuff from 1997 and before. Some of the compatibility features help this even more. Anyone who says otherwise doesn't know what they're talking about.
However, a few things are true, but as steve said, they have been true forever.
First, Windows is bloated. There is a big reason why people install Linux based OS' on older and slower computers. It's because it simply runs faster on them. The argument that high end PC running Linux can do other things such as gaming faster than a Windows based counterpart is certainly valid. Now that has been true for quite awhile. although I would argue that Win 8 might be the most bloated Windows ever. 22 GB of disk space for an OS is a fat horse that needs to trim down. A full install of Ubuntu needs less than 5. I know disk space doesn't equate into performance, but I threw that out there to give people an idea of what I mean in terms of the sheer level of bloat.
Additionally, when building a "gaming" pc, spending $100+ on a copy of Windows is a ton of money that could be spent on a better graphics card. I mean you can almost double the performance for that. Yeah it offers other stuff, but for the majority of gamers I bet steam plus Linux could easily accomplish about any task they desire free of charge. The difference between a $400 and $500 gaming pc really is big.
These advantages start to disappear though for high end machines where the performance difference doesn't really matter and the budget exceeds the $1k mark. However, in the market where the Steam Box will play, those things are huge This is specifically targeting the budget gamer who is looking to spend around $500 (or the cost of a console) to game and wants to consider a PC. Anyone who frequently posts on the Tom's forums knows that about 3 out of every 4 posters looking for build advice fall into that category.