Hello,
Besides support for Replaygain, which has already been discussed (but it's my top-missing feature in Subsonic), a feature that might be nice would be the ability to pick multiple directories for file listing and / or random playing.
That could be achieved by using checkboxes instead of the dropdown list in the left panel, for example (altought it may become messy if the user has a lot of directories...)
This way, the user could choose to play (random) files from directory A and B, but not C. Might be handy when music is organized on a "style per directory" basis.
Don't know if it is more a feature request or a bug report, but it would be cool if the "now playing" view of the center frame could match the currently played song when in random mode (not the small area on the right with the lyrics link but the bigger part of the frame, with the tracklisting of the album and the album art). It currently displays the song that was played at the time the user presses the "top.now_playing" button but does not update when the song changes.
I'm a registered subsonic user, if that somehow matters...