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.
There are some nice new tools emerging to solve this problem, but don't force front end devs into an entirely new platform. One of my projects is an open source server (https://github.com/deployd/deployd) that's totally un-opinionated on the front end, but makes it really easy to add robust backends to client apps.
What I'm saying is that a tool for webdev that wants to stand any chance of becoming popular must be able say to developers: here's the familiar MVC structure, you write your models, you create your views (hopefully out of set of widgets that are highly replaceable/customizable at any moment), we take care of talking to the server, saving your models to the storage and syncing between different instances of an app. Sure, there are problems to solve here, but since everybody does it their own way at the moment, I don't see no reason why we can't have a framework that solves them for you.
