Well, if you read around the Wiki you'll see that rockbox is intended as a music playing firmware. Though doom is supported on some targets, and probably will be on iPod as well given time, its features relate to music. Gapless playback, various new formats (it'll play much higher bitrate Oggs than ipodlinux will, for example).
It also supports a customizeable while playing screen, and several games like Bejeweled, Solitaire, and others.