When you drill down to the lowest level of the media library, you can only play an individual track, you can't queue it. This is especially annoying if you want to create a playlist of an eclectic selection of songs, or you just want to add one song to the end of your play-queue. I'd suggest that perhaps clicking on the track and clicking on the banner that appears on the track should have different actions, where one is queue and one is play, or that it just always queue. It isn't hard to clear the playlist if that's what the user intended (perhaps double-click could be clear/play and every other action queue?) Anyway, I guess this needs some design input, Cc'd Hayat.
Sorry, my previous statement about clearing the playlist is wrong, it doesn't clear the playlist, it just replaces the currently playing song. My bug report is still correct, however (it remains impossible to queue up individual tracks).
After discussing this with Iain we will have Play song replaced with Queue song. On selecting Queue song the song will be added to the playqueue. On selecting anywhere else on the song will start playing the song. This is a solution for the current design. This all needs to be reassessed for 2.2 design.