The library and organizational capabilties of rhythmbox are very cool, and
allow quick and easy access to even a large music library.

Grouping allows you to put together a group of songs for shuffle play, but
doesn't allow you to specify in what order the songs will play, other than
something like track order, alphabetical, or some other organized way. 
Something more closely resembling a traditional playlist could be acheived
by allowing users to reorder songs in a group by dragging and dropping.

That would bring rhythmbox up to the level of XMMS and other music players.
 It's then time to take a step beyond.

Here's my proposal: In addition to groups and the library, support a "Play
queue".  The play queue is a temporary entity -- it's not designed to be
saved or even to stay around for long.  Users could right-click on a song
and choose "Add to Play Queue", which would push the song onto the end of
the play queue.  Some interface (perhaps a sidebar) would allow them to see
the entries in the play queue and reorder them.

Now, when a song ends, rhythmbox would see if there are any songs on the
play queue, in which case a song is popped from the front of the play queue
and played.  If the queue is empty, however, it goes to the next song in
the group/library/album/whatever as though it had never played anything
from the play queue in the first place.

This would allow users to, for example, play in shuffle mode over the whole
library, but if suddenly they decided they wanted to hear a specific song
or set of songs, they could just push them onto the play queue and avoid
having to either wait until the current song ends to do anything or
interrupt the current song.

What do you think of this idea?  I think that it would be a major
improvement in music-player UI.