Rails 5
As has become tradition, a new full version release of Rails is coming just two years after the release of Rails 4. This new version progresses the platform by bumping the underlying minimum Ruby version and adds a slew of neat features to keep Rails feeling fresh and new.Ruby 2.2.2
Ruby 2.2.2 is required in Rails 5 because Rails 5 will take advantage of the new Symbol Garbage Collection found in Ruby 2.2. There is also rumor of Rails 5 using the Incremental Garbage Collection found in Ruby 2.2. They have decided to use Ruby 2.2.2 since Ruby 2.2 had a major security vulnerability that is patched in 2.2.2.
https://github.com/rails/rails/pull/19257 Ruby 2.2.1 PR
https://github.com/rails/rails/commit/32f7491808d2c4e097ed7b3ee875e4d1cea8c442 Ruby 2.2.2 Commit
Rails API
Many Rails developers these days are finding themselves using Javascript Frameworks more and more. Whether DHH likes that or not it's a fact of life. Before Rails 5 developers turned to the ruby-api gem which helps create a minimalist Rails application specifically for use as an API. This functionality is now going to be wrapped up and packaged with Rails 5 so no need for another gem. Just use the command rails new <application name> --api and Rails will create your new API app all on its own!
Here are a couple of tutorials for you Backbone and Ember users.
https://github.com/rails/rails/pull/19832
Turbolinks 3
Turbolinks has been a part of Rails since Rails 4 but is getting a major update to hopefully make developers happier about using it. Turbolinks has been criticized for having major usability problems but the concept of only loading portions of the DOM that change is a sound idea. Many Javascript Frameworks take advantage of this idea specifically React.js. Turbolinks will fetch the body content of your page without worrying about rerendering the CSS and Javascript. You can opt-in to specify which parts of the page should be changed if you'd like as well. They also added a progress bar by default to help the user see things are happening behind the scenes, but with the increased speed you hopefully won't need that.
https://github.com/rails/turbolinks
Action Cable
Action Cable is the feature I am most excited about. Simpler web sockets for Rails. Anytime anyone says web sockets to me I cringe a little just because of how complicated they can be to set up. Many have tried to make the problem easier and Action Cable is Rails' way of giving it a try.
https://github.com/rails/actioncable
Rake or Rails
The beginner's dillema, do I use rake db:migrate or rails db:migrate, is it rake test or rails test? Doesn't matter anymore, it's all rails. From Rails 5 on the rails command can be used to run rake commands. Simple change but a nice one.
https://github.com/rails/rails/issues/18878
Integration Tests
Rails 5 is beginning a push to deprecate Controller tests all together in favor of Integration tests. As part of that they are deprecating assigns() and assert_template in controller tests. Aaron Patterson has a great keynote from Railsconf where he outlines the speed improvements made to the Rails testing environment and why Integration tests will be the way to go.
https://github.com/rails/rails/pull/19058