About breaking backward compatibility - they mentioned at ng-conf an interesting thing today: you'll be able to partially migrate from Angular 1.x to Angular 2 in both ways - having either Angular 1.x app with some Angular 2 components, or Angular 2 app with some Angular 1.x components.

https://www.youtube.com/watch?list=PLOETEcp3DkCoNnlhE-7fovYv...