Basically, as it stands, you have to put your {Ember,Backbone,..} app in `app/assets/javascripts/app/models`, for example, which is kinda crazy. I'd like to see JavaScript become an actual first-class citizen.
Also, the pipeline has been re-written for Rails 4, but historically it's been kinda slow and buggy.
For an example of what I'm thinking about, rake-pipeline-web is interesting.
Rails-api churns out JSON, JS consumes it. I can think of performance being an issue.
What are the other issues?