You can set all MP3/AAC files to open in other players than iTunes you know. Set them all to open in say VLC, then when you select a bunch and double-click they open in VLC as a playlist and start playing. I set all my MP3's to open in QuickTime. If I want to add them to iTunes, I just drag them in, otherwise I open them to play in QuickTime so they don't accidentally open iTunes and add themselves.
QuickLook is for QuickLooking, as someone said, use the previews in Column or Coverflow views to listen to a song without iTunes.
And with Snow Leopard, you can use Icon view. If the icons are big enough you can preview right there, as long as the file stays selected, it'll keep playing in the background.