The asset pipeline which was introduced in Rails 3.1, has been extracted to a separate gem called sprockets-rails. Performance for assets compiling has been improved. There is a open pull request to merge turbo-sprockets-rails3 to sprockets-rails, which would improve its performance further more.
With the commit c82cf81f00f Action and Page caching has been extracted from Action Dispatch as `actionpack-action_caching` and `actionpack-page_caching` gems
With the commit 3324e28804 ActiveRecord::SessionStore is extracted out of Rails into a gem ‘activerecord-session_store’. Setting config.session_store to :active_record_store will no longer work and will break if the activerecord-session_store gem isn’t available.