Not sure if these have been mentioned before, but here are a few of the things i'd love to see in this already fantastic piece of software:
Playlist summary. Things like total run time.. count of tracks in the list etc. If playing a saved list... show the name there and some indication if it's been changed since loaded.
Better last.fm integration. It'd be great if i can 'Love' a song directly from subsonic. Or listen to personal/neighbor radio stations too. Also, i don't think songs should be scrobbled immediately.. shouldn't there be a delay?