According to my iTunes loving Wife... the person I'm trying to convert to Subsonic, her one "Must Have" feature that iTunes does and Subsonic does not do... is the ability to check and uncheck songs in an existing play list. She explains that it allows her to maintain a few playlists, each with a specific mood. But, if she simply wants to exclude a handfull of songs from the list, she needs to only check (or uncheck, not sure which is enable or disable) beside the songs, and they do not show on her iPhone. Yet, the playlist remains intact. So, she can go back later and simply re-enable these songs as needed.
Truth be told, that sounds like a pretty good feature regardless of my motives to get my wife off of iTunes.

Think something like this would be easy to add to Subsonic?
Cheers
Shane.