I'm still unsure with what your infatuation with Linux here is. As mentioned, Linux is a kernel that is in use for lots of totally random and different things. The userland (stuff outside the kernel) is what is interesting and the different aspect, it's the main piece of software that separates the sniper rifle from the supercomputer.
The problem with Linux as a gaming platform has nothing to do with the design of the kernel. It has everything to do with the fact that games for desktops are binary compatible with Windows, but by-and-large are not binary compatible with Linux.
Steam isn't in a position to take DirectX/Windows binary products like CoD and make them run binary-compatibly and fast on Linux, whereas they probably could make games designed to run on normal Linux work straight out of the box. The problem is that DX/Win is where the money is, and so that's what developers write games for.
If Steambox doesn't run AAA games out-of-the-box and within days of Launch on other platforms, it'll never be more than an also-ran.