Cog user since I converted to the Mac, Feb 2007.
Freeing me from the artificial complexity of a media library is what makes me use Cog... I organize my own files as I'm putting them into my Music folder, I don't need my hand held by iTunes. I also hate iTunes' interface.
I'd like to see a different interface for Cog myself though. Something that has the player controls as miniaturized as possible, maybe make them tuck up underneath the menubar, with detachable playlists like winamp, and like Audion, the ability to have more than one playlist open at once. Also, never fall into the trap of making a skinned only gui... you probably won't, given this is Mac OS, and people don't usually want that type of feature, but if you do implement some sort of skinning, always remember to leave an option to use the native GUI.