It's always a trade off. The more you customise the less compatible you are with other things, the harder it is for OEMs to comply etc. It's a sliding scale though, you just have to match the implementation to a business model.
I tend to think Valve will put something standard (ish maybe customised graphics drivers) on there and have totally normal hardware. It'll be built on common standards and basically compatible with existing hardware. My hunch anyway. One obvious gain is that if it runs normal 'nix it's automatically a better media hub than any console. I'll be able to have VLC playing MKV files for one.
They're just expanding their user base a bit really. It's not such a big story.