It'd be great if Subsonic could somehow visually distinguish which albums have been listened to per user -- perhaps with a mark or symbol next to the album in the left menu (something subtle would probably be best) or in the "recently added" view (since going to this view implies a user who is waiting for new music to be added).
At the moment, my users and I have a system of each "starring" everything added to Subsonic so that we have a list of albums we know we have to listen to, and then we un-star after we've listened. Obviously inefficient and clutters what is otherwise a great feature.