Rails dropped its dependency on jQuery, rewriting all of it's unobtrusive JavaScript methods in pure JS. This change will be released as part of Rails 5.1.
Rails will support Webpack and Yarn as of 5.1, this is exciting to me as it should clear up a common pain point of pulling in JavaScript libraries and keeping them up to date in an easy way. Now we should be able to update JavaScript just like gems.
Rails 5 includes view helpers into controllers, so that they're available for Action Cable to publish messages from the controller. Rails 4 doesn't include helpers by default. The PushType CMS that I've been working with is Rails 4 and 5 compatible, and this was the first bug I ran into for Rails 4 support. It made the assumption that the helpers will be available. To fix, I had to include them into the needed class manually as part of my initialization script.