I can't speak to the Backbone.js changes, but isn't one of the odd things about Rails versioning the fact that 3.x -> 4.0 is essentially a dot release upgrade, but 4.0 to 4.1 a full version upgrade (many things are deprecated from 3.x to 4.0, and 4.1 will remove them entirely?)
That was the plan, but now we are planning to make 4.1 an easy upgrade as well, and not remove deprecations that were originally planned on being removed.
