Couple that with my skepticism that they're actually shipping much if any traditional Linux userspace software, and it's even less useful to compare potential SteamOS users to traditional Linux users.
Other than that I agree that with Valve as a patron there's a lot more credibility. Steam was a long-term move, and it paid off. So Valve has a good track record and solid momentum. If they can help make it easy for game companies to port their work, taking out a significant or non-trivial amount of work and offer the carrot of sales & paying customers, I could see this taking off.
I was skeptical of Steam back in 2004 but I wasn't thinking long term enough. Launching your own operating system isn't a short term move. It suggests to me that this will only get better and more impressive over time, if Valve is as committed to this as it seems.