f6bbc3f5 adds app/models/concerns and app/controllers/concerns to the load path. This is the change that supports the officially blessed way to put fat models on a diet.
Rails supports the PostgreSQL int4range and int8range data types directly as of 9a4a095e.
Support for email obfuscation was removed from the mail_to helper in cf9d9450. There's a new actionview-encoded_mail_to gem if you still need it.
86c5cea9 cleans up some of the documentation on controller-specific assets, a perennial source of confusion.
Built-in background jobs support was stripped out of master at f9da785d; it'll be worked on in a branch and won't ship with Rails 4.0.