It has a "Railsier" "Convention over Configuration" type ethos. Certainly more opinionated, if nothing else, where as Backbone is more of an empty canvas & instruments to cover it.

There are technical difference as well, but I think that's less of an issue.

I'd say Backbone is a library. Ember is more of a framework.

For example, create 100 todos in their demo app, then check the select all button. Notice the instance response. This is because Ember consolidate actions and executes them once at the end of the event loop. This is not possible in Backbone without basically reimplementing that functionality.

I believe this is what they mean by "ambitious," because it's designed around issues that will arise in an ambitious Backbone project.

Backbone:Sinatra as Ember:Rails for a rough analogy.