Considering the Xbox One is running on what's basically PC hardware, and they already have the dashboard running off Windows 10 software, getting Steam to run on the Xbox One would probably be trivial... but wont' happen because MS wouldn't forfeit that much control over the console's storefront.
For cross platform MS wants devs to use the universal app platform sdk, and they will have full control over what apps are allowed onto Xbox, and which apps will require a Gold sub. It'll still make things way easier for devs to get their apps on Xbox, vs other consoles.