- Templates: Allows your new skeleton Rails application to be built your way with your default stack of gems, configs, and more.
- Engines: Share reusable application pieces complete with routes that Just Work, models, view paths, and the works.
- Rack: Rails now runs on Rack which gives you access to all the middleware goodness.
- Metal: Write super fast pieces of optimized logic that routes around Action Controller.
- Nested forms: Deal with complex forms so much easier.