I am only interested in these features:

- you need access to the GPU stack, which is running in kernel-mode, btw, to output video and get hw acceleration;

- you need access to the audio stack, also in low-level mode;

- you need access to the DSP acceleration (not always the GPU);

- you need access to the system settings to disable screensavers, and adjust brightness;

- you need to unzip, untar, decrypt, decipher and so on;

- many OpenGL client libraries need access to the /etc too;

Is there a lighter version where these features are cut?

- you need to be able to open files without user interactions (no file picker), in order to open playlist, MXF or MKV files;

- you need the same if ever you have a database of files (media center oriented);

- you need raw access to /dev/* to play DVD, CD and other optical disk (and the equivalent on Windows);

- you need ioctl on such devices, to pass the MMC for DVD/Bluray;

- you need raw access to /dev/v4l* for your webcams and be able to control them;

- on linux, you have access to x11 for the 3 above features, which is almost root;

- you need access to /etc/ (registry) for proxy informations, fonts configuration and accessibility;

- you need access to the network, as input and output (think remote control);

- you need access to mounts to be able to see the insertion of DVD/Bluray/USB/SD cards and such;

- you need to expose an IPC (think MPRIS on Linux);

- you need access to the fonts and the fonts configuration (see fontconfig).