The backwards compatibility is technically not an issue if MS is going to pull a "Steam" on us. In other words, as far as i understand, Microsoft wants to have you register every game on your Live Account via some sort of "code". This also provides the ability to play the game without the disc, and MS is probably going to force you to install to the HD anyway.
So based on that, it's no different than Steam, which will merely do an online check to validate the game, maybe once a week or whenever you're online.
Catch is, as mentioned, if you want to sell your disc, you can't, it's like trying to sell a Steam game. HOWEVER, the advantage is, you can RE-download the game directly from MS if you ever need to play it in the future. So the disc is just a way to save some bandwidth.
So what am i getting at? There's your backward compatibility. If you can somehow register all your Xbox 360 games on your Live Account, implying you don't need the disc, then what stops you from downloading (from the Live Store) and playing it on the Xbox One ? I'm sure MS will "port" or "convert" some of the games across and sell it in their store as digital games?