You're forgetting the advantage that consoles have. Well defined hardware that software can be optimized specifically. They will get a boost from this effect.Depends on the hardware. I can build a $1500 Steam machine that can play everything at full 1080p (for most hi-def TV's), but I can also build a $500 Steam machine that will play a lot of stuff, but either not at full res or with ultra detail.
I like my PC, and I like consoles, but if I can play the same on my PC, I'll do it there. If there are Steam Box exclusive games, I'll definitely look into it. But, why buy an inferior machine when I can have the exact same thing on my desk? It seems like it's a lower powered PC running Steam. I have a higher powered PC running Steam. I'm good.
Not saying it's a bad product, I just wouldn't be a target consumer for this device.