Now, to begin, I would like to first point out that I do own multiple Android powered devices, but none which are mentioned here in this post, so please don't cry "fanboy" as I have 0 allegiance to anything except Android. And yes, I'm kicking myself for not getting in on the Ouya kickstarter (even though I had multiple chances).
Ouya is poised to become the next big multimedia console to enter the consumer market and home, and it's got quite the uphill battle ahead of it. There's already plenty of small low-powered Android devices either announced or already released which fill one niche or another. While Ouya markets itself primarily as a game console, the fact that it runs Android means that it's got to compete with every other low-powered Android device which feasibly can accomplish everything the Ouya can. I'll expound on this further.
Ouya vs ...:
Google TV. Starting at the same price point for the VIZIO Co-Star, this device provides more than enough power in a small frame to power your perfect TV setup, providing internet access as well as local network streaming for your entertainment needs. Now, while the Google TV platform is marketed as a STB, it's still a competitor in mild/moderate gaming as well as web content accessibility. With the fact that Google TV is synonymous with "everything Google, now on your TV," Ouya's name will lend to confusion as to what it really is for the mass market in the beginning, hurting intial adoption rates outside the Android community.
Win: Google TV, brand recognition.
Raspberry Pi. Starting at a paltry $25, this little low-powered Android stock device is actually quite a surprising little power house. All manner of network appliances have been developed around this hardware, and with the drivers for most of the hardware being provided for other flavors of Linux, it's range and scope is expanding fast. While again, only techies will really know what the Pi is, it's heavily marketed (ignore the fact i'm using this term loosely) towards Android and computing enthusiasts as a replacement for all those things that are big, hot, and noisy. This little gem has already received more builds of Linux than I can count, a port of XBMC that can easily handle streaming 1080p without a sweat, it really comes down to accessibility. In the long run the Ouya is pricier, and for those just looking for a cheap XBMC device, you can't beat the Pi at $25. That and it's kawaii-small.
Win: Raspberry Pi, price point.
nVIDIA Shield. In the closest thing to apples to apples comparison of devices based on how they're marketed, we have the nVIDIA Shield, the Tegra 4 powered nVIDIA Android handheld gaming console (announced). This little gamer's wet dream is a powerhouse in your hands, and throw in the ability to play your PC games on the handheld thanks to special integration with the nVIDIA graphics processor on your PC, and you've got an almost universal system to enjoy anywhere, anytime. Again, being that it's Android, don't expect that it won't be without it's ports of XBMC and many many other wonderful pieces of software to further enhance the cost-to-value ratio of this handheld. Being that this is the closest competitor to the Ouya, it's worth noting that there are a few caveats to the Shield which bring it down. As of this writing, the "Play PC" feature is heavily Steam oriented (not a bad thing), will likely require Multi-Band Wireless N (MIMO) (not prevalent, likely have to buy one), and the biggest bullet to chew on, a whopping GeForce GTX 650 (cheapest on Newegg as of writing $110 new) in order to enjoy this device to it's fullest. While the Ouya lacks this functionality to begin with, it brings it down, but this feature feels more like a power-play by nVIDIA than something that could end up becoming mainstream.
Win: Ouya, will integrate with everything you already have provided tools and/or apps are provided to link it, no need to upgrade everything around it to make full use of it.
Mods: Sorry for the perceived dupe topic, I was at work getting calls every 15 minutes interrupting me for upwards half an hour after I started writing this just after lunch.