I think the Ouya dev team have shot themselves in the foot by omitting the Play Store. Sure it can be sideloaded, but I don't think most people would go through the trouble of doing that. I was interested in this device, but because it is not a handheld and there are so many other up and coming developments happening in the Android gaming world (mostly from JXD and Yinlips) I'll probably pass.
I know this comment is old now, but Piston is a bit of a joke as it costs $999 and doesn't even run the games very well. Valve is supposedly working on their own box that might be better, but only time will tell.