Actually, it doesn't "force" you to use get/set. You can just POJOs just fine w/ Ember, but you won't get property observation or unknownProperty support.
I also question your assumption that Object.observe will be as fast as Ember's observation system. I wouldn't be surprised if it was several times slower.
Object.observe also doesn't support array observation, which is something we support in Ember.
That's true. Ember isn't trying to provide a set of UI widgets out of the box. I'm confident that Ember-compatible UI libraries will be released in the near future.
SproutCore 1.x isn't very mobile-friendly at the moment. The original reason the SproutCore 2.0 rewrite began was to make the framework lightweight so that it'd be more suitable for use on mobile platforms.
I've started a project, Ember-Titanium, that enables native mobile app development using Ember's runtime on the Titanium Mobile platform. The project is still in it's infancy, but I've already been successful at building apps with it.
I also question your assumption that Object.observe will be as fast as Ember's observation system. I wouldn't be surprised if it was several times slower.
Object.observe also doesn't support array observation, which is something we support in Ember.