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
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.
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.
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.
A lot of other under the hood improvements are expected to be made but I think I covered a lot of the major upcoming features. Let me know if you have any questions or which Rails feature you are most excited about by leaving a comment below.