There's a few caveats to this BC...
First off, not all games are available immediately. Secondly, what about DLC? I bought a lot of DLC for Black Ops. Are my purchases going to be transferable to my One? Thirdly, what about full digital games that I purchased on my 360? I have a copy of Burnout: 3 that I got digitally. Will that be able to transfer to my One some day? Lastly, what about Arcade games?
Don't get me wrong, I'm not saying that BC is a BAD thing, but I'm afraid people aren't going to wait to get all the facts and will be lining up to sell their 360s and then later get a rude awakening to some of these circumstances that they didn't think of.