What makes me sad about js client-side frameworks and Ember.js in particular is that they are still only client-side. Nevermind the code duplication, but the bare fact that I have to actually think about how to construct my server so that it works with Ember.js. The reason why people hooked up on Rails so quickly is because it solved all of their problems, not just one. Novices are not going to try Ember.js just because it works on the client, while having no idea about the server-side.