Don't compress file types known to be already compressed
Files types that are already compressed (eg mp4, mp3, zip) should just be stored rather than compressed.
There should be a checkbox labelled something like "avoid compressing these extensions" and when user clicks that checkbox a text field appears, into which the user can type a comma separated list of file extensions, and any files with that extension don't get compressed.
Use case: I often find myself zipping up large folders with a mix of many types of files, some highly compressible, and others already compressed (video files, music files, other archives, etc). This feature would greatly speed up creating such an archive.