It seems that Apple constantly makes it difficult to queue songs on the fly, meaning that users can add songs to the playlist without interfering with the song currently playing. It took a long time before such a feature was even included in either iTunes of iPod and the implementations are satisfactory at best.
The iTunes’ take on the issue is the so called Party Shuffle feature. One item on the sidebar acts as the party shuffle playlist, where people can insert songs by dragging them from the library. The problem is that the feature is hardly intuitive for the users who have not seen it before. Dragging can also be considered tedious by some but no keyboard shortcut is provided. It is illustrative that the feature is so hard to figure out that Apple has decided to explain its usage with an OK dialog.
In a party context it is common that guests want to add their favorite songs to the playlist but I have never seen it happen that users would respect the Party Shuffle mode and act accordingly. What usually happens is that people end up using the library for browsing the songs and either accidentally or purposefully double-click them, ending the song currently playing.
Nowadays iTunes also boasts a full screen Cover Flow view which looks great but is rather useless. Certainly, it would be great to have a kiosk-like full screen interface that makes the task of managing songs easy for novices and looks great while doing it. The current full screen view lets one pick a song but does not provide any means of doing this without ending the one currently playing. I spent some time with Keynote and put together a little mock-up that demonstrates how the full screen view could be made more useful by adding a queue function akin to the Party Shuffle. The flipping CD covers are borrowed from the iPhone and do not necessarily directly fit to a mouse-driven environment.
(It seems that YouTube somewhat messed up the synchronization of audio and video)
This video only shows how one can manually manage the playlist. Shoud the list become empty, it could automatically be filled, like the current Party Shuffle does. A search function would be helpful, too, but this has not been done in the video. Moreover, it would be useful to show the titles of the songs on the playlist when hovering on the album covers with the mouse but this has not been implemented due to the limitations of Keynote. It can be argued that it is misleading to use album covers to represent individual songs but Apple has done this in other areas of iTunes as well as on the iPhone, so this should be well in line with the existing design decisions.
(The Cover Flow view of iPhone is equally disappointing. By its nature, it is highly suitable for going through the music collection, looking for an inspiration for the song to play but the inability to select songs without ending the currently played makes it rather unusable for this purpose. The only way that iPhone lets one add songs to playlists is through a cumbersome list, which is only sortable by the song-name.)