The Title Tag
Now is unnecessary wp_title(); formerly included in header.php. Instead, use add_theme_support( 'title-tag' ); which is included in the file functionality of the theme.

Read more Title Tags in 4.1 at

Archive Title Template Tags
Instead get_the_archive_title() now the_archive_title()

New Post Navigations
Can see the new numbered navigation in action in the new default theme Twenty Fifteen. This function original new, before you had to use the WordPress pagination plugins to get the same result.

No Header Page and Background Page in the Admin
Now redirected directly to the Customizer for setting Header or Background Image. File custom-header.php included in the theme but without detailed instructions.

Some changes we implemented in themes Planum Free, Oprum and Rectangulum.