If XBMC wants to run with hardware acceleration then that will probably require extra effort to make sure that there are drivers it can work with.
1) Controller Support 2) Must have at least a free demo, if not another free component.
Also it looks like they won't be having the Play Store, meaning that for many (most?) Android title won't work with Ouya.
[0] XBMC is mostly custom, but happens it hails from a controller-based platform to begin with.
So this is more like an "Angry Birds is coming to Kindle Fire" thing. Just because it's compatible, doesn't mean it will be available out of the box, if it doesn't have access to the Play Store, and since it's a much different form factor from phones and tablets, the interfaces will need to be changed quite a bit.
It's really hard to use on apple tv with the remote. So very unintuitive. And they are also heavily invested in their theme engine - we'll have an "android-looking theme" at best, not native UI...
