Rails form generation helper, "form_with" defaulted to remote forms previously. This would confuse beginners who would expect an HTML response to be rendered. Now, you can configure the default response type.
With Rails adding support for check constraints to database migrations, check constraints can easily now be added and removed from tables with simple migration DSL. Earlier, the only way to add check constraints to tables was through raw SQL queries.
Destroying multiple layered hierarchy of associated objects can be time consuming and might result in a server timeout error. With Rails adding support for dependent associations to be deleted in the background, this issue is addressed.