What Valve should do:
- Transparent cross-platform build with Valve tools
The dev shouldn't have to invest time or energy even thinking about if it is worth his while to support Mac/Linux. It should be no more than a button press to build for all three insofar as is possible.
- Open bug tracker so people can search for issues, raise them, provide feedback, and track them
Witness the blank cliff-face that is ATI/AMDs propriety driver support (admittedly this may be better now, but I haven't heard of an official bug tracker, just the unofficial one)
- Put pressure on Nvidia to provide docs to the nouveau guys without NDA's
Encouragement toward AMD on their sterling efforts so far, and outright congrats towards Intel, who seem to be the most enlightened and productive. Intel do not just work on their own drivers, but (AFAICT) work on the core infrastructure too.
- Build a package for every distro
But by the same token, don't make it an immense pain to get running elsewhere because of bad choices. Distro Community feedback should be heeded, because they are the experts.
- Write drivers, infrastructure, compositors, operating system level stuff
This is not their core mission, and getting dragged into this will reduce the focus from where it is needed: The client, the tools, and the games.