Note: This project is currently a Trac ticket that has been committed for 4.6 (#36753) and is available for testing in trunk/nightlies.
When a visual and small screen admin refresh was introduced in 3.8 (by way of the feature plugin MP6), the admin font was changed to Open Sans to better complement the redesigned vector iconography. This change was not without its bumps or controversy, notably around extended character sets and that it is loaded from Google Fonts for a variety of reasons.
Instead of relying on an external resource, Font Natively moves the WordPress admin back to system fonts. This leads to faster load times, especially when working offline, a removal of a third-party dependency, and a more native-feeling experience as the lines between web experiences and apps continue to blur.
Development – A plugin is available on GitHub by @mattmiklic and is ready for testing. It is also available in patch form on #36753.
Documentation & Testing – This needs to be tested for any misalignments or other issues when switching to system fonts.
Example screenshots (more needed)
Open Sans, Firefox, OSX
System fonts, Firefox, OSX
System fonts, Firefox, Arch Linux