During the development of Rails 4, many features that were present in earlier versions of Rails were removed from Rails itself and extracted to gems. Extracting features to gems slims down Rails itself, and allows Rails to take a different direction to solve certain problems. For example, strong_…

This article has a good list of Rails features that will be extracted into gems.

Link: Upgrading Rails: Gems Extracted in Rails 4 — Andy Lindeman via alindeman.github.com