What I fear is that Valve will dive deep into Linux, and then suddenly realize that supporting software like steam and games on Linux may be a bit more challenging than they thought it would.
The myriad hardware types out there with myriad sets of less-than-optimal drivers might present myriad problems, even if Valve does master the video-card/opengl end of things. I know I get vastly different experiences with Ubuntu depending on if I install it on one desktop versus another versus my laptop. They all have their own sets of issues, and none of them are remotely perfect.
This whole affair with valve just reminds me of some computer user adopting a new platform with vim and vigor...and then realizing it's not all it's cracked up to be a few weeks or months later. I myself did this with mac, but it took a couple years for me to come to my senses, unfortunately.
There are MANY legitimate reasons why Linux on the desktop has not taken off. I fear that Valve just hasn't encountered the right set of those reasons yet.