libusb - Use virtual/libusb for usb device hotplug support. This flag should only be enabled if you're running a non-Linux kernel or you don't want to use udev.
lirc - Enable support for IR remote controls using app-misc/lirc
mariadb - Enable support store of media library metadata on local or remote MariaDB
mysql - Enable support store of media library metadata on local or remote MySQL
optical - Enable Audio CD playback, optical disks detection and reading (CD-ROM, Video CD, and others), and ISO optical disk images direct reading. DVD disks may require additional 'udf' flag.
samba - Add support for SAMBA (Windows File and Printer sharing)
system-ffmpeg - Use system ffmpeg instead of the bundled one
test - Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
udev - Use udev rules to handle devices' permissions and hotplug support. Unless you know what you're doing do not disable this flag on Linux kernels. This is provided as an option for completeness.
udf - Enable UDF support. Required for playing blurays.
udisks - Enable storage management support (automounting, volume monitoring, etc)